Fixes <http://bugs.gnu.org/24886>.
Previously, the grafting derivation of, say, brdf-explorer would pull in
qt:doc even though brdf-explorer depends only on qt:out, not qt:doc.
* guix/grafts.scm (with-cache): Use 'vhash-assoc' and 'vhash-cons'
instead of 'vhash-assq' and 'vhash-consq'.
(cumulative-grafts): Pass #:outputs to 'graft-derivation/shallow'. Use
OUTPUTS instead of (derivation-output-names drv).
(graft-derivation): Add #:outputs parameter; pass it to
'cumulative-grafts'.
* tests/grafts.scm (make-derivation-input): New variable.
("graft-derivation, replaced derivation has multiple outputs"): Make
sure P2:zzz is not part of the outputs of P3D.
("graft-derivation with #:outputs")
("graft-derivation, unused outputs not depended on"): New tests.
