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.

lisp/lib/debug.el

@@ -331,7 +331,7 @@ ready to be pasted in a bug report on github."
          ,@(condition-case e
                (mapcar
                 #'cdr (doom--collect-forms-in
-                       (doom-path doom-user-dir "packages.el")
+                       (doom-path doom-user-dir doom-module-packages-file)
                        "package!"))
              (error (format "<%S>" e))))
         (unpin
@@ -339,7 +339,7 @@ ready to be pasted in a bug report on github."
                (mapcan #'identity
                        (mapcar
                         #'cdr (doom--collect-forms-in
-                               (doom-path doom-user-dir "packages.el")
+                               (doom-path doom-user-dir doom-module-packages-file)
                                "unpin!")))
              (error (list (format "<%S>" e)))))
         (elpa