emacs: Factorize searching for Emacs packages.

* emacs/guix-emacs.el: (guix-emacs-directories): New procedure.
(guix-emacs-find-autoloads-in-directory): Rename to...
(guix-emacs-find-autoloads): ... this.
(guix-emacs-load-autoloads): Remove.  Replace with...
(guix-emacs-autoload-packages): ... this.  New procedure.  At first,
find a list of directories with Emacs packages, then add them to
'load-path' and finally, load autoloads.
* doc/emacs.texi (Emacs Initial Setup): Adjust accordingly.