emacs: Move the code to load Emacs packages after installing.

This code should belong to "guix-backend" as it is run after a REPL
operation to check if new Emacs packages have been installed and to
autoload them if needed.

* emacs/guix-emacs.el (guix-emacs-activate-after-operation)
(guix-emacs-load-autoloads-maybe): Move and rename to...
* emacs/guix-backend.el (guix-emacs-activate-after-operation)
(guix-repl-autoload-emacs-packages-maybe): ... this.
(guix-after-repl-operation-hook): Adjust for the renaming.