import: hackage: Support recursive importing.

* guix/import/hackage.scm (hackage-recursive-import): New procedure.
(hackage-module->sexp): Return dependencies alongside dependencies.
(hackage->guix-package): Memoize results.
* guix/scripts/import/hackage.scm (show-help, %options, guix-import-hackage):
Support recursive importing.
* doc/guix.texi (Invoking guix import): Document option.