tpl/tplimpl: Rework template management to get rid of concurrency issues

This more or less completes the simplification of the template handling code in Hugo started in v0.62.

The main motivation was to fix a long lasting issue about a crash in HTML content files  without front matter.

But this commit also comes with a big functional improvement.

As we now have moved the base template evaluation to the build stage we now use the same lookup rules for `baseof` as for `list` etc. type of templates.

This means that in this simple example you can have a `baseof` template for the `blog` section without having to duplicate the others:

```
layouts
├── _default
│   ├── baseof.html
│   ├── list.html
│   └── single.html
└── blog
    └── baseof.html
```

Also, when simplifying code, you often get rid of some double work, as shown in the "site building" benchmarks below.

These benchmarks looks suspiciously good, but I have repeated the below with ca. the same result. Compared to master:

```
name                              old time/op    new time/op    delta
SiteNew/Bundle_with_image-16        13.1ms ± 1%    10.5ms ± 1%  -19.34%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    13.0ms ± 0%    10.7ms ± 1%  -18.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      46.4ms ± 2%    43.1ms ± 1%   -7.15%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            52.2ms ± 2%    47.8ms ± 1%   -8.30%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        77.9ms ± 1%    70.9ms ± 1%   -9.01%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      43.0ms ± 0%    37.2ms ± 1%  -13.54%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         58.2ms ± 1%    52.4ms ± 1%   -9.95%  (p=0.029 n=4+4)

name                              old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image-16        3.81MB ± 0%    2.22MB ± 0%  -41.70%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16    3.60MB ± 0%    2.01MB ± 0%  -44.20%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16      19.3MB ± 1%    14.1MB ± 0%  -26.91%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16            70.7MB ± 0%    69.0MB ± 0%   -2.40%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16        37.1MB ± 0%    31.2MB ± 0%  -15.94%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16      17.6MB ± 0%    10.6MB ± 0%  -39.92%  (p=0.029 n=4+4)
SiteNew/Page_collections-16         25.9MB ± 0%    21.2MB ± 0%  -17.99%  (p=0.029 n=4+4)

name                              old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image-16         52.3k ± 0%     26.1k ± 0%  -50.18%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16     52.3k ± 0%     26.1k ± 0%  -50.16%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16        336k ± 1%      269k ± 0%  -19.90%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16              422k ± 0%      395k ± 0%   -6.43%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16          401k ± 0%      313k ± 0%  -21.79%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16        247k ± 0%      143k ± 0%  -42.17%  (p=0.029 n=4+4)
SiteNew/Page_collections-16           282k ± 0%      207k ± 0%  -26.55%  (p=0.029 n=4+4)
```

Fixes #6716
Fixes #6760
Fixes #6768
Fixes #6778

1 files changed, 9 insertions, 3 deletions

diff --git a/tpl/template.go b/tpl/template.go
index 0841236de..b9b0749b6 100644
--- a/tpl/template.go
+++ b/tpl/template.go
@@ -30,9 +30,7 @@ type TemplateManager interface {
 	TemplateFuncGetter
 	AddTemplate(name, tpl string) error
 	AddLateTemplate(name, tpl string) error
-	LoadTemplates(prefix string) error
-
-	RebuildClone()
+	MarkReady() error
 }
 
 // TemplateVariants describes the possible variants of a template.
@@ -52,6 +50,8 @@ type TemplateFinder interface {
 type TemplateHandler interface {
 	TemplateFinder
 	Execute(t Template, wr io.Writer, data interface{}) error
+	LookupLayout(d output.LayoutDescriptor, f output.Format) (Template, bool, error)
+	HasTemplate(name string) bool
 }
 
 type TemplateLookup interface {
@@ -105,6 +105,12 @@ type templateInfoManager struct {
 	InfoManager
 }
 
+// TemplatesProvider as implemented by deps.Deps.
+type TemplatesProvider interface {
+	Tmpl() TemplateHandler
+	TextTmpl() TemplateParseFinder
+}
+
 // WithInfo wraps the info in a template.
 func WithInfo(templ Template, info Info) Template {
 	if manager, ok := info.(InfoManager); ok {