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/template_test.go

@@ -20,6 +20,7 @@ import (
 	"testing"
 
 	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/config/allconfig"
 	"github.com/gohugoio/hugo/identity"
 
 	qt "github.com/frankban/quicktest"
@@ -30,9 +31,10 @@ import (
 
 func TestTemplateLookupOrder(t *testing.T) {
 	var (
-		fs  *hugofs.Fs
-		cfg config.Provider
-		th  testHelper
+		fs      *hugofs.Fs
+		cfg     config.Provider
+		th      testHelper
+		configs *allconfig.Configs
 	)
 
 	// Variants base templates:
@@ -189,7 +191,8 @@ func TestTemplateLookupOrder(t *testing.T) {
 		t.Run(this.name, func(t *testing.T) {
 			// TODO(bep) there are some function vars need to pull down here to enable => t.Parallel()
 			cfg, fs = newTestCfg()
-			th = newTestHelper(cfg, fs, t)
+			this.setup(t)
+			th, configs = newTestHelperFromProvider(cfg, fs, t)
 
 			for i := 1; i <= 3; i++ {
 				writeSource(t, fs, filepath.Join("content", fmt.Sprintf("sect%d", i), fmt.Sprintf("page%d.md", i)), `---
@@ -199,9 +202,7 @@ Some content
 `)
 			}
 
-			this.setup(t)
-
-			buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
+			buildSingleSite(t, deps.DepsCfg{Fs: fs, Configs: configs}, BuildCfg{})
 			// helpers.PrintFs(s.BaseFs.Layouts.Fs, "", os.Stdout)
 			this.assert(t)
 		})