Previously we'd make 502 'query-valid-derivers' RPCs for
"guix build vim -d", and after this patch, we don't do any.
Furthermore, the previous strategy was "stateful" in the sense that
'item->deriver' could return a derivation that is not the one that was
actually computed by this process, but an "equivalent" one (due to
fixed-output derivations); which one is chosen would depend on the state
of the store.
This in turn means that we'd have to call 'read-derivation-from-file' to
actually read .drv files (as opposed to getting them from
%DERIVATION-CACHE). This is costly and doesn't work with
GUIX_DAEMON_SOCKET=ssh://….
* guix/grafts.scm (item->deriver): Remove.
(reference-origin): New procedure.
(cumulative-grafts): Use it instead of 'item->deriver'.
aad086d871