* guix/import/elpa.scm (latest-release): Determine the repository based
on the URL of the source.
(package-from-gnu.org?): Rename to ...
(elpa-repository): ...this; memoize.
(package-from-elpa-repository?): New procedure.
(%elpa-updater): Adjust accordingly.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>