refactor: rethink doom-module-*-path functions

- Rename doom-module-path -> doom-module-expand-path, to better reflect
  its purpose.
- Optimize doom-module-locate-path to try caches and
  locate-file-internal, before looping through doom-modules-dirs.
- Rely on file-name-concat to join paths, rather than string
  concatenation. file-name-concat is more robust for the purpose and
  has lower overhead than expand-file-name.

bin/doom

@@ -295,9 +295,9 @@ SEE ALSO:
     (let ((cli-file "cli"))
       (defcli-group! "Module commands"
         (dolist (key (hash-table-keys doom-modules))
-          (when-let (path (plist-get (gethash key doom-modules) :path))
+          (when-let (path (doom-module-expand-path (car key) (cdr key) cli-file))
             (defcli-group! :prefix (format "+%s" (cdr key))
-              (load! cli-file path t)))))
+              (doom-load path t)))))
 
       (load! cli-file doom-user-dir t))