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/doom-start.el

@@ -318,7 +318,7 @@ If RETURN-P, return the message as a string instead of displaying it."
 
 ;;; Load $DOOMDIR/init.el early
 ;; TODO: Catch errors
-(doom-load (file-name-concat doom-user-dir doom-module-init-file) t)
+(load! (string-remove-suffix ".el" doom-module-init-file) doom-user-dir t)
 
 ;;; Load the rest of $DOOMDIR + modules if noninteractive
 ;; If the user is loading this file from a batch script, let's assume they want