hugofs: Make FileMeta a struct

This commit started out investigating a `concurrent map read write` issue, ending by replacing the map with a struct.

This is easier to reason about, and it's more effective:

```
name                                  old time/op    new time/op    delta
SiteNew/Regular_Deep_content_tree-16    71.5ms ± 3%    69.4ms ± 5%    ~     (p=0.200 n=4+4)

name                                  old alloc/op   new alloc/op   delta
SiteNew/Regular_Deep_content_tree-16    29.7MB ± 0%    27.9MB ± 0%  -5.82%  (p=0.029 n=4+4)

name                                  old allocs/op  new allocs/op  delta
SiteNew/Regular_Deep_content_tree-16      313k ± 0%      303k ± 0%  -3.35%  (p=0.029 n=4+4)
```

See #8749

1 files changed, 17 insertions, 15 deletions

diff --git a/hugofs/filter_fs.go b/hugofs/filter_fs.go
index bee4d05d2..9da63bbb7 100644
--- a/hugofs/filter_fs.go
+++ b/hugofs/filter_fs.go
@@ -44,7 +44,7 @@ func NewLanguageFs(langs map[string]int, fs afero.Fs) (afero.Fs, error) {
 			}
 
 			meta := fi.(FileMetaInfo).Meta()
-			lang := meta.Lang()
+			lang := meta.Lang
 
 			fileLang, translationBaseName, translationBaseNameWithExt := langInfoFrom(langs, fi.Name())
 			weight := 0
@@ -58,14 +58,16 @@ func NewLanguageFs(langs map[string]int, fs afero.Fs) (afero.Fs, error) {
 				lang = fileLang
 			}
 
-			fim := NewFileMetaInfo(fi, FileMeta{
-				metaKeyLang:                       lang,
-				metaKeyWeight:                     weight,
-				metaKeyOrdinal:                    langs[lang],
-				metaKeyTranslationBaseName:        translationBaseName,
-				metaKeyTranslationBaseNameWithExt: translationBaseNameWithExt,
-				metaKeyClassifier:                 files.ClassifyContentFile(fi.Name(), meta.GetOpener()),
-			})
+			fim := NewFileMetaInfo(
+				fi,
+				&FileMeta{
+					Lang:                       lang,
+					Weight:                     weight,
+					Ordinal:                    langs[lang],
+					TranslationBaseName:        translationBaseName,
+					TranslationBaseNameWithExt: translationBaseNameWithExt,
+					Classifier:                 files.ClassifyContentFile(fi.Name(), meta.OpenFunc),
+				})
 
 			fis[i] = fim
 		}
@@ -74,9 +76,9 @@ func NewLanguageFs(langs map[string]int, fs afero.Fs) (afero.Fs, error) {
 	all := func(fis []os.FileInfo) {
 		// Maps translation base name to a list of language codes.
 		translations := make(map[string][]string)
-		trackTranslation := func(meta FileMeta) {
-			name := meta.TranslationBaseNameWithExt()
-			translations[name] = append(translations[name], meta.Lang())
+		trackTranslation := func(meta *FileMeta) {
+			name := meta.TranslationBaseNameWithExt
+			translations[name] = append(translations[name], meta.Lang)
 		}
 		for _, fi := range fis {
 			if fi.IsDir() {
@@ -90,9 +92,9 @@ func NewLanguageFs(langs map[string]int, fs afero.Fs) (afero.Fs, error) {
 
 		for _, fi := range fis {
 			fim := fi.(FileMetaInfo)
-			langs := translations[fim.Meta().TranslationBaseNameWithExt()]
+			langs := translations[fim.Meta().TranslationBaseNameWithExt]
 			if len(langs) > 0 {
-				fim.Meta()["translations"] = sortAndremoveStringDuplicates(langs)
+				fim.Meta().Translations = sortAndremoveStringDuplicates(langs)
 			}
 		}
 	}
@@ -108,7 +110,7 @@ func NewFilterFs(fs afero.Fs) (afero.Fs, error) {
 	applyMeta := func(fs *FilterFs, name string, fis []os.FileInfo) {
 		for i, fi := range fis {
 			if fi.IsDir() {
-				fis[i] = decorateFileInfo(fi, fs, fs.getOpener(fi.(FileMetaInfo).Meta().Filename()), "", "", nil)
+				fis[i] = decorateFileInfo(fi, fs, fs.getOpener(fi.(FileMetaInfo).Meta().Filename), "", "", nil)
 			}
 		}
 	}