gnu: emacs: Adapt the autoloads auxiliary code to use EMACSLOADPATH.

gnu: emacs: Adapt the autoloads auxiliary code to use EMACSLOADPATH.

The Elisp directories to scan for autoloads are now taken from EMACSLOADPATH
instead of from the user profile, environment profile or system profile.
Manually adding the Elisp directories to the `load-path' is no longer
necessary, as this is covered by Emacs when they are in EMACSLOADPATH.  The
caching logic is also removed, as this code is not typically run often and the
gain is marginal (loading autoloads files is cheap).

* gnu/packages/aux-files/emacs/guix-emacs.el (guix-emacs-directory)
(guix-emacs-subdirs, guix-emacs-directories): Remove procedures.
(guix-emacs-find-autoloads): Filter the directory entries by passing the
regexp to `directory-files' directly, which is ten times faster.  Remove
deduplication.
(guix-emacs-autoloads-regexp): Remove the group, which used to filter out the
file extension; it no longer works this way due to passing the regexp to the
`directory-files' procedure directly, which doesn't care about groups.
(guix-emacs-autoload-packages): Update doc.  Search package directories from
EMACSLOADPATH.  Do not populate the load-path.  Remove cache.