packages: 'package-grafts' returns grafts for all the relevant outputs.

packages: 'package-grafts' returns grafts for all the relevant outputs.

Fixes <https://bugs.gnu.org/41796>.
Reported by Jakub Kądziołka <kuba@kadziolka.net>.

* guix/packages.scm (input-graft): Add 'output' parameter and honor it.
Add OUTPUT to the cache key.
(input-cross-graft): Likewise.
(fold-bag-dependencies): Operate on inputs instead of nodes.  Turn
VISITED into a vhash instead of a set.  Pass PROC HEAD and OUTPUT
instead of just HEAD.
(bag-grafts): Adjust accordingly.
* tests/packages.scm ("package-grafts, dependency on several outputs"):
New test.