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

testscripts/commands/mod_tidy.txt

@@ -0,0 +1,21 @@
+# Test hugo mod tidy.
+
+dostounix golden/go.mod.cleaned
+
+hugo mod tidy
+
+cmp go.mod golden/go.mod.cleaned
+
+-- hugo.toml --
+title = "Hugo Modules Test"
+-- go.mod --
+go 1.19
+
+require github.com/bep/empty-hugo-module v1.0.0
+
+module github.com/gohugoio/testmod
+-- golden/go.mod.cleaned --
+go 1.19
+
+
+module github.com/gohugoio/testmod