grafts: Simplify access to store item references.

grafts: Simplify access to store item references.

This is a followup to 710854304b1ab29332edcb76f3de532e0724c197.

This also slightly reduces the number of 'query-references' RPCs, for
instance from 176 to 166 from "guix build emacs -d".

* guix/grafts.scm (references-oracle): Remove.
(non-self-references): Remove 'references' parameter and add 'store'.
Add 'references*' procedure and use it instead of 'references'.  Adjust
caller accordingly.
(cumulative-grafts): Remove 'references' parameter and adjust caller
accordingly.