refactor: change doom-module-list signature

Used to return the hash-table `doom-modules` (if not all-p), but I've
changed it to return a list of cons cells (:CATEGORY . MODULE),
representing all enabled modules, in the order they were enabled.

The purpose of this change is to prepare for a change in the structure
of doom-modules, and how Doom stores its module metadata.

lisp/lib/help.el

@@ -342,8 +342,7 @@ without needing to check if they are available."
       (describe-function fn))))
 
 (defun doom--help-modules-list ()
-  (cl-loop for path in (cdr (doom-module-load-path 'all))
-           for (cat . mod) = (doom-module-from-path path)
+  (cl-loop for (cat . mod) in (doom-module-list 'all)
            for readme-path = (or (doom-module-locate-path cat mod "README.org")
                                  (doom-module-locate-path cat mod))
            for format = (format "%s %s" cat mod)