import: stackage: Support recursive importing.

* guix/import/hackage.scm (hackage-name->package-name): Export procedure.
* guix/import/stackage.scm (lts-info-packages-lts-info): Fix match expression.
(stackage-recursive-import): New procedure.
(stackage->guix-package): Memoize results.
* guix/scripts/import/stackage.scm (show-help, %options,
guix-import-stackage): Support recursive importing.
* doc/guix.texi (Invoking guix import): Document option.