Introduce a tree map for all content

This commit introduces a new data structure to store pages and their resources.

This data structure is backed by radix trees.

This simplies tree operations, makes all pages a bundle,  and paves the way for #6310.

It also solves a set of annoying issues (see list below).

Not a motivation behind this, but this commit also makes Hugo in general a little bit faster and more memory effective (see benchmarks). Especially for partial rebuilds on content edits, but also when taxonomies is in use.

```
name                                   old time/op    new time/op    delta
SiteNew/Bundle_with_image/Edit-16        1.32ms ± 8%    1.00ms ± 9%  -24.42%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16    1.28ms ± 0%    0.94ms ± 0%  -26.26%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      33.9ms ± 2%    21.8ms ± 1%  -35.67%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            40.6ms ± 1%    37.7ms ± 3%   -7.20%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        56.7ms ± 0%    51.7ms ± 1%   -8.82%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      19.9ms ± 2%    18.3ms ± 3%   -7.64%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         37.9ms ± 4%    34.0ms ± 2%  -10.28%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             10.7ms ± 0%    10.6ms ± 0%   -1.15%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         10.8ms ± 0%    10.7ms ± 0%   -1.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16           43.2ms ± 1%    39.6ms ± 1%   -8.35%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 47.6ms ± 1%    47.3ms ± 0%     ~     (p=0.057 n=4+4)
SiteNew/Deep_content_tree-16             73.0ms ± 1%    74.2ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Many_HTML_templates-16           37.9ms ± 0%    38.1ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Page_collections-16              53.6ms ± 1%    54.7ms ± 1%   +2.09%  (p=0.029 n=4+4)

name                                   old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image/Edit-16         486kB ± 0%     430kB ± 0%  -11.47%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     265kB ± 0%     209kB ± 0%  -21.06%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      13.6MB ± 0%     8.8MB ± 0%  -34.93%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            66.5MB ± 0%    63.9MB ± 0%   -3.95%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        28.8MB ± 0%    25.8MB ± 0%  -10.55%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      6.16MB ± 0%    5.56MB ± 0%   -9.86%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         16.9MB ± 0%    16.0MB ± 0%   -5.19%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             2.28MB ± 0%    2.29MB ± 0%   +0.35%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         2.07MB ± 0%    2.07MB ± 0%     ~     (p=0.114 n=4+4)
SiteNew/Tags_and_categories-16           14.3MB ± 0%    13.2MB ± 0%   -7.30%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 69.1MB ± 0%    69.0MB ± 0%     ~     (p=0.343 n=4+4)
SiteNew/Deep_content_tree-16             31.3MB ± 0%    31.8MB ± 0%   +1.49%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16           10.8MB ± 0%    10.9MB ± 0%   +1.11%  (p=0.029 n=4+4)
SiteNew/Page_collections-16              21.4MB ± 0%    21.6MB ± 0%   +1.15%  (p=0.029 n=4+4)

name                                   old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image/Edit-16         4.74k ± 0%     3.86k ± 0%  -18.57%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     4.73k ± 0%     3.85k ± 0%  -18.58%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16        301k ± 0%      198k ± 0%  -34.14%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16              389k ± 0%      373k ± 0%   -4.07%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16          338k ± 0%      262k ± 0%  -22.63%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16        102k ± 0%       88k ± 0%  -13.81%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16           176k ± 0%      152k ± 0%  -13.32%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16              26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16          26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16             273k ± 0%      245k ± 0%  -10.36%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                   396k ± 0%      398k ± 0%   +0.39%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16               317k ± 0%      325k ± 0%   +2.53%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16             146k ± 0%      147k ± 0%   +0.98%  (p=0.029 n=4+4)
SiteNew/Page_collections-16                210k ± 0%      215k ± 0%   +2.44%  (p=0.029 n=4+4)
```

Fixes #6312
Fixes #6087
Fixes #6738
Fixes #6412
Fixes #6743
Fixes #6875
Fixes #6034
Fixes #6902
Fixes #6173
Fixes #6590

5 files changed, 33 insertions, 12 deletions

diff --git a/hugofs/decorators.go b/hugofs/decorators.go
index 123655ba0..6247f6183 100644
--- a/hugofs/decorators.go
+++ b/hugofs/decorators.go
@@ -80,7 +80,8 @@ func DecorateBasePathFs(base *afero.BasePathFs) afero.Fs {
 
 // NewBaseFileDecorator decorates the given Fs to provide the real filename
 // and an Opener func.
-func NewBaseFileDecorator(fs afero.Fs) afero.Fs {
+func NewBaseFileDecorator(fs afero.Fs, callbacks ...func(fi FileMetaInfo)) afero.Fs {
+
 	ffs := &baseFileDecoratorFs{Fs: fs}
 
 	decorator := func(fi os.FileInfo, filename string) (os.FileInfo, error) {
@@ -120,7 +121,14 @@ func NewBaseFileDecorator(fs afero.Fs) afero.Fs {
 			return ffs.open(filename)
 		}
 
-		return decorateFileInfo(fi, ffs, opener, filename, "", meta), nil
+		fim := decorateFileInfo(fi, ffs, opener, filename, "", meta)
+
+		for _, cb := range callbacks {
+			cb(fim)
+		}
+
+		return fim, nil
+
 	}
 
 	ffs.decorate = decorator
diff --git a/hugofs/fileinfo.go b/hugofs/fileinfo.go
index f5e95e952..79d89a88b 100644
--- a/hugofs/fileinfo.go
+++ b/hugofs/fileinfo.go
@@ -39,6 +39,7 @@ const (
 
 	metaKeyBaseDir                    = "baseDir" // Abs base directory of source file.
 	metaKeyMountRoot                  = "mountRoot"
+	metaKeyModule                     = "module"
 	metaKeyOriginalFilename           = "originalFilename"
 	metaKeyName                       = "name"
 	metaKeyPath                       = "path"
@@ -100,10 +101,10 @@ func (f FileMeta) Name() string {
 	return f.stringV(metaKeyName)
 }
 
-func (f FileMeta) Classifier() string {
-	c := f.stringV(metaKeyClassifier)
-	if c != "" {
-		return c
+func (f FileMeta) Classifier() files.ContentClass {
+	c, found := f[metaKeyClassifier]
+	if found {
+		return c.(files.ContentClass)
 	}
 
 	return files.ContentClassFile // For sorting
@@ -131,6 +132,10 @@ func (f FileMeta) MountRoot() string {
 	return f.stringV(metaKeyMountRoot)
 }
 
+func (f FileMeta) Module() string {
+	return f.stringV(metaKeyModule)
+}
+
 func (f FileMeta) Weight() int {
 	return f.GetInt(metaKeyWeight)
 }
diff --git a/hugofs/files/classifier.go b/hugofs/files/classifier.go
index 9aa2476b7..e8f8241b7 100644
--- a/hugofs/files/classifier.go
+++ b/hugofs/files/classifier.go
@@ -49,14 +49,20 @@ func IsContentExt(ext string) bool {
 	return contentFileExtensionsSet[ext]
 }
 
+type ContentClass string
+
 const (
-	ContentClassLeaf    = "leaf"
-	ContentClassBranch  = "branch"
-	ContentClassFile    = "zfile" // Sort below
-	ContentClassContent = "zcontent"
+	ContentClassLeaf    ContentClass = "leaf"
+	ContentClassBranch  ContentClass = "branch"
+	ContentClassFile    ContentClass = "zfile" // Sort below
+	ContentClassContent ContentClass = "zcontent"
 )
 
-func ClassifyContentFile(filename string) string {
+func (c ContentClass) IsBundle() bool {
+	return c == ContentClassLeaf || c == ContentClassBranch
+}
+
+func ClassifyContentFile(filename string) ContentClass {
 	if !IsContentFile(filename) {
 		return ContentClassFile
 	}
diff --git a/hugofs/filter_fs.go b/hugofs/filter_fs.go
index 952b276cf..a42cd233a 100644
--- a/hugofs/filter_fs.go
+++ b/hugofs/filter_fs.go
@@ -185,7 +185,7 @@ func (fs *FilterFs) Open(name string) (afero.File, error) {
 }
 
 func (fs *FilterFs) OpenFile(name string, flag int, perm os.FileMode) (afero.File, error) {
-	panic("not implemented")
+	return fs.fs.Open(name)
 }
 
 func (fs *FilterFs) ReadDir(name string) ([]os.FileInfo, error) {
diff --git a/hugofs/rootmapping_fs.go b/hugofs/rootmapping_fs.go
index 229f25fc6..283bb042d 100644
--- a/hugofs/rootmapping_fs.go
+++ b/hugofs/rootmapping_fs.go
@@ -65,6 +65,7 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 
 		rm.Meta[metaKeyBaseDir] = rm.ToBasedir
 		rm.Meta[metaKeyMountRoot] = rm.path
+		rm.Meta[metaKeyModule] = rm.Module
 
 		meta := copyFileMeta(rm.Meta)
 
@@ -121,6 +122,7 @@ type RootMapping struct {
 	From      string   // The virtual mount.
 	To        string   // The source directory or file.
 	ToBasedir string   // The base of To. May be empty if an absolute path was provided.
+	Module    string   // The module path/ID.
 	Meta      FileMeta // File metadata (lang etc.)
 
 	fi   FileMetaInfo