emacs: Add interface for package locations.

* emacs/guix-main.scm (%package-location-param-alist): New variable.
(package-location->sexp, package-location-entries): New procedures.
* emacs/guix-ui-location.el: New file.
* emacs.am (ELFILES): Add it.
* doc/emacs.texi (Emacs Package Locations): Document 'guix-locations'.
* NEWS: Mention it.