grafts: Allow the replacement to have a different name.

* guix/build/graft.scm (replace-store-references): REPLACEMENT is now
the full string, not just the hash.
(rewrite-directory)[hash-mapping](valid-suffix?): Remove.
(hash+suffix): Rename to...
(hash+rest): ... this.  Change to return the whole string as the second
element of the list.  Adjust 'match-lambda' expression accordingly;
check whether the string length of the origin and replacement match.
* tests/grafts.scm ("graft-derivation, grafted item uses a different
name"): New test.
* doc/guix.texi (Security Updates): Update sentence on the name/version
restriction.