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

modules/client.go

@@ -433,9 +433,9 @@ func (c *Client) Clean(pattern string) error {
 		if g != nil && !g.Match(m.Path) {
 			continue
 		}
-		_, err = hugofs.MakeReadableAndRemoveAllModulePkgDir(c.fs, m.Dir)
+		dirCount, err := hugofs.MakeReadableAndRemoveAllModulePkgDir(c.fs, m.Dir)
 		if err == nil {
-			c.logger.Printf("hugo: cleaned module cache for %q", m.Path)
+			c.logger.Printf("hugo: removed %d dirs in module cache for %q", dirCount, m.Path)
 		}
 	}
 	return err