Simplify page tree logic

This is preparation for #6041.

For historic reasons, the code for bulding the section tree and the taxonomies were very much separate.

This works, but makes it hard to extend, maintain, and possibly not so fast as it could be.

This simplification also introduces 3 slightly breaking changes, which I suspect most people will be pleased about. See referenced issues:

This commit also switches the radix tree dependency to a mutable implementation: github.com/armon/go-radix.

Fixes #6154
Fixes #6153
Fixes #6152

hugolib/page__common.go

@@ -30,6 +30,8 @@ type pageCommon struct {
 	s *Site
 	m *pageMeta
 
+	bucket *pagesMapBucket
+
 	// Laziliy initialized dependencies.
 	init *lazy.Init
 
@@ -101,17 +103,17 @@ type pageCommon struct {
 	translationKey     string
 	translationKeyInit sync.Once
 
-	// Will only be set for sections and regular pages.
+	// Will only be set for bundled pages.
 	parent *pageState
 
-	// Will only be set for section pages and the home page.
-	subSections page.Pages
-
 	// Set in fast render mode to force render a given page.
 	forceRender bool
 }
 
 type pagePages struct {
-	pages     page.Pages
 	pagesInit sync.Once
+	pages     page.Pages
+
+	regularPagesInit sync.Once
+	regularPages     page.Pages
 }