Before that, outputs of a derivation could be left referring to the
ungrafted version of the derivation.
* guix/grafts.scm (graft-derivation)[outputs]: Change to a list of
name/file pairs.
* guix/grafts.scm (graft-derivation)[build]: Add 'old-outputs' variable
and use it when computing 'mapping'. Use 'mapping' directly.
* tests/grafts.scm ("graft-derivation, multiple outputs"): New test.
f376dc3acb