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/rss_test.go

@@ -23,24 +23,22 @@ import (
 
 func TestRSSOutput(t *testing.T) {
 	t.Parallel()
-	var (
-		cfg, fs = newTestCfg()
-		th      = newTestHelper(cfg, fs, t)
-	)
 
 	rssLimit := len(weightedSources) - 1
 
-	rssURI := "index.xml"
-
+	cfg, fs := newTestCfg()
 	cfg.Set("baseURL", "http://auth/bub/")
 	cfg.Set("title", "RSSTest")
 	cfg.Set("rssLimit", rssLimit)
+	th, configs := newTestHelperFromProvider(cfg, fs, t)
+
+	rssURI := "index.xml"
 
 	for _, src := range weightedSources {
 		writeSource(t, fs, filepath.Join("content", "sect", src[0]), src[1])
 	}
 
-	buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
+	buildSingleSite(t, deps.DepsCfg{Fs: fs, Configs: configs}, BuildCfg{})
 
 	// Home RSS
 	th.assertFileContent(filepath.Join("public", rssURI), "<?xml", "rss version", "RSSTest")