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

config/allconfig/integration_test.go

@@ -0,0 +1,71 @@
+package allconfig_test
+
+import (
+	"path/filepath"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/config/allconfig"
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestDirsMount(t *testing.T) {
+
+	files := `
+-- hugo.toml --
+baseURL = "https://example.com"
+disableKinds = ["taxonomy", "term"]
+[languages]
+[languages.en]
+weight = 1
+[languages.sv]
+weight = 2
+[[module.mounts]]
+source = 'content/en'
+target = 'content'
+lang = 'en'
+[[module.mounts]]
+source = 'content/sv'
+target = 'content'
+lang = 'sv'
+-- content/en/p1.md --
+---
+title: "p1"
+---
+-- content/sv/p1.md --
+---
+title: "p1"
+---
+-- layouts/_default/single.html --
+Title: {{ .Title }}
+	`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{T: t, TxtarString: files},
+	).Build()
+
+	//b.AssertFileContent("public/p1/index.html", "Title: p1")
+
+	sites := b.H.Sites
+	b.Assert(len(sites), qt.Equals, 2)
+
+	configs := b.H.Configs
+	mods := configs.Modules
+	b.Assert(len(mods), qt.Equals, 1)
+	mod := mods[0]
+	b.Assert(mod.Mounts(), qt.HasLen, 8)
+
+	enConcp := sites[0].Conf
+	enConf := enConcp.GetConfig().(*allconfig.Config)
+
+	b.Assert(enConcp.BaseURL().String(), qt.Equals, "https://example.com")
+	modConf := enConf.Module
+	b.Assert(modConf.Mounts, qt.HasLen, 2)
+	b.Assert(modConf.Mounts[0].Source, qt.Equals, filepath.FromSlash("content/en"))
+	b.Assert(modConf.Mounts[0].Target, qt.Equals, "content")
+	b.Assert(modConf.Mounts[0].Lang, qt.Equals, "en")
+	b.Assert(modConf.Mounts[1].Source, qt.Equals, filepath.FromSlash("content/sv"))
+	b.Assert(modConf.Mounts[1].Target, qt.Equals, "content")
+	b.Assert(modConf.Mounts[1].Lang, qt.Equals, "sv")
+
+}