guix: opam: Add --repo argument to importer.

* guix/scripts/import/opam.scm (guix-import-opam): Pass --repo argument
to recursive and non-recursive importers.
* guix/import/opam.scm (get-opam-repository): Select proper repository
location depending on a new repo argument.
(opam->guix-package): Use get-opam-repository in the procedure body.