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

deps/deps.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"go.uber.org/atomic"
 
 	"github.com/gohugoio/hugo/cache/filecache"
 	"github.com/gohugoio/hugo/common/loggers"
@@ -38,10 +39,10 @@ type Deps struct {
 	DistinctWarningLog *helpers.DistinctLogger
 
 	// The templates to use. This will usually implement the full tpl.TemplateManager.
-	Tmpl tpl.TemplateHandler `json:"-"`
+	tmpl tpl.TemplateHandler
 
 	// We use this to parse and execute ad-hoc text templates.
-	TextTmpl tpl.TemplateParseFinder `json:"-"`
+	textTmpl tpl.TemplateParseFinder
 
 	// The file systems to use.
 	Fs *hugofs.Fs `json:"-"`
@@ -92,6 +93,9 @@ type Deps struct {
 	// BuildStartListeners will be notified before a build starts.
 	BuildStartListeners *Listeners
 
+	// Atomic flags set during a build.
+	BuildFlags BuildFlags
+
 	*globalErrHandler
 }
 
@@ -153,9 +157,20 @@ type ResourceProvider interface {
 	Clone(deps *Deps) error
 }
 
-// TemplateHandler returns the used tpl.TemplateFinder as tpl.TemplateHandler.
-func (d *Deps) TemplateHandler() tpl.TemplateManager {
-	return d.Tmpl.(tpl.TemplateManager)
+func (d *Deps) Tmpl() tpl.TemplateHandler {
+	return d.tmpl
+}
+
+func (d *Deps) TextTmpl() tpl.TemplateParseFinder {
+	return d.textTmpl
+}
+
+func (d *Deps) SetTmpl(tmpl tpl.TemplateHandler) {
+	d.tmpl = tmpl
+}
+
+func (d *Deps) SetTextTmpl(tmpl tpl.TemplateParseFinder) {
+	d.textTmpl = tmpl
 }
 
 // LoadResources loads translations and templates.
@@ -315,6 +330,7 @@ func (d Deps) ForLanguage(cfg DepsCfg, onCreated func(d *Deps) error) (*Deps, er
 	}
 
 	d.BuildStartListeners = &Listeners{}
+	d.BuildFlags = BuildFlags{}
 
 	return &d, nil
 
@@ -358,3 +374,14 @@ type DepsCfg struct {
 	// Whether we are in running (server) mode
 	Running bool
 }
+
+// BuildFlags are flags that may be turned on during a build.
+type BuildFlags struct {
+	HasLateTemplate atomic.Bool
+}
+
+func NewBuildFlags() BuildFlags {
+	return BuildFlags{
+		//HasLateTemplate: atomic.NewBool(false),
+	}
+}