refactor: use doom-module-*-file variables; add two

- Adds doom-module-packages-file and doom-module-metadata-file.
- Uses them and the other doom-module-*-file variables where they were
  previously hardcoded.
- Add .el extension to doom-module-{init,config}-file; it is now the
  consumer's responsibility to strip/change/keep the extension as they
  see fit.

modules/config/literate/autoload.el

@@ -51,7 +51,7 @@
   "Tangles `+literate-config-file' if it has changed."
   (or (getenv "__NOTANGLE")
       (and (+literate-tangle +literate-config-file
-                             (concat doom-module-config-file ".el")
+                             doom-module-config-file
                              doom-user-dir)
            (or (not noninteractive)
                (exit! "__NOTANGLE=1 $@")))))
@@ -80,7 +80,7 @@
                          (prin1-to-string
                           `(funcall #',(symbol-function #'+literate-tangle)
                                     ,+literate-config-file
-                                    ,(concat doom-module-config-file ".el")
+                                    ,doom-module-config-file
                                     ,doom-user-dir))))
     (add-hook 'kill-emacs-hook #'+literate-tangle-check-finished-h)
     (set-process-sentinel +literate-tangle--async-proc #'+literate-tangle--async-sentinel)