Fixes <http://bugs.gnu.org/23718>.
Reported by Efraim Flashner <efraim@flashner.co.il>.
* guix/gnu-maintenance.scm (find-packages): Remove.
(find-package): New procedure.
* guix/import/gnu.scm (gnu->guix-package): Use 'find-package' instead of
'find-packages' and adjust accordingly.