emacs: Add 'guix-packages-by-location' command.

* emacs/guix-main.scm (packages-by-location-file, package-location-files):
New procedures.
(%patterns-makers): Add 'location' search type.
* emacs/guix-messages.el (guix-message-packages-by-location): New procedure.
(guix-messages): Use it.
* emacs/guix-read.el (guix-package-locations)
(guix-read-package-location): New procedures.
* emacs/guix-ui-package.el (guix-packages-by-location): New command.
* doc/emacs.texi (Emacs Commands): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>