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/site_sections_test.go

@@ -137,21 +137,20 @@ PAG|{{ .Title }}|{{ $sect.InSection . }}
 		}},
 		{"empty1", func(assert *require.Assertions, p page.Page) {
 			// > b,c
-			assert.NotNil(getPage(p, "/empty1/b"))
+			assert.Nil(getPage(p, "/empty1/b")) // No _index.md page.
 			assert.NotNil(getPage(p, "/empty1/b/c"))
 
 		}},
 		{"empty2", func(assert *require.Assertions, p page.Page) {
-			// > b,c,d where b and d have content files.
+			// > b,c,d where b and d have _index.md files.
 			b := getPage(p, "/empty2/b")
 			assert.NotNil(b)
 			assert.Equal("T40_-1", b.Title())
+
 			c := getPage(p, "/empty2/b/c")
+			assert.Nil(c) // No _index.md
 
-			assert.NotNil(c)
-			assert.Equal("Cs", c.Title())
 			d := getPage(p, "/empty2/b/c/d")
-
 			assert.NotNil(d)
 			assert.Equal("T41_-1", d.Title())
 
@@ -163,9 +162,10 @@ PAG|{{ .Title }}|{{ $sect.InSection . }}
 		{"empty3", func(assert *require.Assertions, p page.Page) {
 			// b,c,d with regular page in b
 			b := getPage(p, "/empty3/b")
-			assert.NotNil(b)
-			assert.Len(b.Pages(), 1)
-			assert.Equal("empty3.md", b.Pages()[0].File().LogicalName())
+			assert.Nil(b) // No _index.md
+			e3 := getPage(p, "/empty3/b/empty3")
+			assert.NotNil(e3)
+			assert.Equal("empty3.md", e3.File().LogicalName())
 
 		}},
 		{"empty3", func(assert *require.Assertions, p page.Page) {
@@ -188,19 +188,23 @@ PAG|{{ .Title }}|{{ $sect.InSection . }}
 		}},
 		{"l1", func(assert *require.Assertions, p page.Page) {
 			assert.Equal("L1s", p.Title())
-			assert.Len(p.Pages(), 2)
+			assert.Len(p.Pages(), 4) // 2 pages + 2 sections
 			assert.True(p.Parent().IsHome())
 			assert.Len(p.Sections(), 2)
 		}},
 		{"l1,l2", func(assert *require.Assertions, p page.Page) {
 			assert.Equal("T2_-1", p.Title())
-			assert.Len(p.Pages(), 3)
+			assert.Len(p.Pages(), 4) // 3 pages + 1 section
 			assert.Equal(p, p.Pages()[0].Parent())
 			assert.Equal("L1s", p.Parent().Title())
 			assert.Equal("/l1/l2/", p.RelPermalink())
 			assert.Len(p.Sections(), 1)
 
 			for _, child := range p.Pages() {
+				if child.IsSection() {
+					assert.Equal(child, child.CurrentSection())
+					continue
+				}
 
 				assert.Equal(p, child.CurrentSection())
 				active, err := child.InSection(p)