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

hugolib/hugo_sites.go

@@ -121,6 +121,9 @@ type hugoSitesInit struct {
 	// Loads the data from all of the /data folders.
 	data *lazy.Init
 
+	// Performs late initialization (before render) of the templates.
+	layouts *lazy.Init
+
 	// Loads the Git info for all the pages if enabled.
 	gitInfo *lazy.Init
 
@@ -130,6 +133,7 @@ type hugoSitesInit struct {
 
 func (h *hugoSitesInit) Reset() {
 	h.data.Reset()
+	h.layouts.Reset()
 	h.gitInfo.Reset()
 	h.translations.Reset()
 }
@@ -271,6 +275,7 @@ func newHugoSites(cfg deps.DepsCfg, sites ...*Site) (*HugoSites, error) {
 		Sites:        sites,
 		init: &hugoSitesInit{
 			data:         lazy.New(),
+			layouts:      lazy.New(),
 			gitInfo:      lazy.New(),
 			translations: lazy.New(),
 		},
@@ -289,6 +294,15 @@ func newHugoSites(cfg deps.DepsCfg, sites ...*Site) (*HugoSites, error) {
 		return nil, nil
 	})
 
+	h.init.layouts.Add(func() (interface{}, error) {
+		for _, s := range h.Sites {
+			if err := s.Tmpl().(tpl.TemplateManager).MarkReady(); err != nil {
+				return nil, err
+			}
+		}
+		return nil, nil
+	})
+
 	h.init.translations.Add(func() (interface{}, error) {
 		if len(h.Sites) > 1 {
 			allTranslations := pagesToTranslationsMap(h.Sites)
@@ -429,10 +443,6 @@ func NewHugoSites(cfg deps.DepsCfg) (*HugoSites, error) {
 
 func (s *Site) withSiteTemplates(withTemplates ...func(templ tpl.TemplateManager) error) func(templ tpl.TemplateManager) error {
 	return func(templ tpl.TemplateManager) error {
-		if err := templ.LoadTemplates(""); err != nil {
-			return err
-		}
-
 		for _, wt := range withTemplates {
 			if wt == nil {
 				continue
@@ -619,10 +629,10 @@ func (h *HugoSites) renderCrossSitesArtifacts() error {
 
 	s := h.Sites[0]
 
-	smLayouts := []string{"sitemapindex.xml", "_default/sitemapindex.xml", "_internal/_default/sitemapindex.xml"}
+	templ := s.lookupLayouts("sitemapindex.xml", "_default/sitemapindex.xml", "_internal/_default/sitemapindex.xml")
 
 	return s.renderAndWriteXML(&s.PathSpec.ProcessingStats.Sitemaps, "sitemapindex",
-		s.siteCfg.sitemap.Filename, h.toSiteInfos(), smLayouts...)
+		s.siteCfg.sitemap.Filename, h.toSiteInfos(), templ)
 }
 
 func (h *HugoSites) removePageByFilename(filename string) {
@@ -832,7 +842,7 @@ func (h *HugoSites) resetPageStateFromEvents(idset identity.Identities) {
 				if po.cp == nil {
 					continue
 				}
-				for id, _ := range idset {
+				for id := range idset {
 					if po.cp.dependencyTracker.Search(id) != nil {
 						po.cp.Reset()
 						continue OUTPUTS
@@ -841,7 +851,7 @@ func (h *HugoSites) resetPageStateFromEvents(idset identity.Identities) {
 			}
 
 			for _, s := range p.shortcodeState.shortcodes {
-				for id, _ := range idset {
+				for id := range idset {
 					if idm, ok := s.info.(identity.Manager); ok && idm.Search(id) != nil {
 						for _, po := range p.pageOutputs {
 							if po.cp != nil {