Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

hugolib/page_permalink_test.go

@@ -16,12 +16,11 @@ package hugolib
 import (
 	"fmt"
 	"html/template"
-	"path/filepath"
 	"testing"
 
 	qt "github.com/frankban/quicktest"
 
-	"github.com/gohugoio/hugo/deps"
+	"github.com/gohugoio/hugo/config"
 )
 
 func TestPermalink(t *testing.T) {
@@ -68,28 +67,38 @@ func TestPermalink(t *testing.T) {
 		t.Run(fmt.Sprintf("%s-%d", test.file, i), func(t *testing.T) {
 			t.Parallel()
 			c := qt.New(t)
-			cfg, fs := newTestCfg()
-
+			cfg := config.New()
 			cfg.Set("uglyURLs", test.uglyURLs)
 			cfg.Set("canonifyURLs", test.canonifyURLs)
-			cfg.Set("baseURL", test.base)
 
-			pageContent := fmt.Sprintf(`---
+			files := fmt.Sprintf(`
+-- hugo.toml --
+baseURL = %q
+-- content/%s --
+---
 title: Page
 slug: %q
-url: %q
+url: %q	
 output: ["HTML"]
 ---
-Content
-`, test.slug, test.url)
+`, test.base, test.file, test.slug, test.url)
 
-			writeSource(t, fs, filepath.Join("content", filepath.FromSlash(test.file)), pageContent)
+			if i > 0 {
+				t.Skip()
+			}
 
-			s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{SkipRender: true})
+			b := NewIntegrationTestBuilder(
+				IntegrationTestConfig{
+					T:           t,
+					TxtarString: files,
+					BaseCfg:     cfg,
+				},
+			)
+
+			b.Build()
+			s := b.H.Sites[0]
 			c.Assert(len(s.RegularPages()), qt.Equals, 1)
-
 			p := s.RegularPages()[0]
-
 			u := p.Permalink()
 
 			expected := test.expectedAbs