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.txt

@@ -0,0 +1,44 @@
+# Test the hugo mod commands.
+
+dostounix golden/vendor.txt
+dostounix golden/go.mod.testsubmod
+
+hugo mod graph
+stdout 'empty-hugo'
+hugo mod verify
+! stderr .
+hugo mod get -u
+! stderr .
+hugo mod get -u ./...
+! stderr .
+hugo mod vendor
+! stderr .
+cmp _vendor/modules.txt golden/vendor.txt
+hugo mod clean
+! stderr .
+stdout 'hugo: removed 1 dirs in module cache for \"github.com/bep/empty-hugo-module\"'
+hugo mod clean --all
+stdout 'Deleted 2\d{2} files from module cache\.'
+cd submod
+hugo mod init testsubmod
+cmpenv go.mod $WORK/golden/go.mod.testsubmod
+-- hugo.toml --
+title = "Hugo Modules Test"
+[module]
+[[module.imports]]
+path="github.com/bep/empty-hugo-module"
+[[module.imports.mounts]]
+source="README.md"
+target="content/_index.md"
+-- go.mod --
+go 1.19
+
+module github.com/gohugoio/testmod
+-- submod/hugo.toml --
+title = "Hugo Sub Module"
+-- golden/vendor.txt --
+# github.com/bep/empty-hugo-module v1.0.0
+-- golden/go.mod.testsubmod --
+module testsubmod
+
+go ${GOVERSION}