Reimplement and simplify Hugo's template system

See #13541 for details.

Fixes #13545
Fixes #13515
Closes #7964
Closes #13365
Closes #12988
Closes #4891

resources/page/page.go

@@ -150,8 +150,8 @@ type InSectionPositioner interface {
 
 // InternalDependencies is considered an internal interface.
 type InternalDependencies interface {
-	// GetRelatedDocsHandler is for internal use only.
-	GetRelatedDocsHandler() *RelatedDocsHandler
+	// GetInternalRelatedDocsHandler is for internal use only.
+	GetInternalRelatedDocsHandler() *RelatedDocsHandler
 }
 
 // OutputFormatsProvider provides the OutputFormats of a Page.

resources/page/page_paths.go

@@ -145,7 +145,7 @@ func CreateTargetPaths(d TargetPathDescriptor) (tp TargetPaths) {
 		pb.isUgly = true
 	}
 
-	if d.Type == output.HTTPStatusHTMLFormat || d.Type == output.SitemapFormat || d.Type == output.RobotsTxtFormat {
+	if d.Type == output.HTTPStatus404HTMLFormat || d.Type == output.SitemapFormat || d.Type == output.RobotsTxtFormat {
 		pb.noSubResources = true
 	} else if d.Kind != kinds.KindPage && d.URL == "" && d.Section.Base() != "/" {
 		if d.ExpandedPermalink != "" {

resources/page/pages_related.go

@@ -129,7 +129,7 @@ func (p Pages) withInvertedIndex(ctx context.Context, search func(idx *related.I
 		return nil, fmt.Errorf("invalid type %T in related search", p[0])
 	}
 
-	cache := d.GetRelatedDocsHandler()
+	cache := d.GetInternalRelatedDocsHandler()
 
 	searchIndex, err := cache.getOrCreateIndex(ctx, p)
 	if err != nil {

resources/page/testhelpers_test.go

@@ -221,7 +221,7 @@ func (p *testPage) GetTerms(taxonomy string) Pages {
 	panic("testpage: not implemented")
 }
 
-func (p *testPage) GetRelatedDocsHandler() *RelatedDocsHandler {
+func (p *testPage) GetInternalRelatedDocsHandler() *RelatedDocsHandler {
 	return relatedDocsHandler
 }