emacs: Handle autoloading Emacs packages installed with Guix.

* emacs/guix-emacs.el: New file.
* emacs.am (ELFILES): Add it.
* emacs/guix-backend.el (guix-after-repl-operation-hook): Add
  'guix-emacs-load-autoloads-maybe'.
* emacs/guix-init.el.in: Autoload emacs packages.
  (guix-package-enable-at-startup): New variable.