grafts: 'name' parameter of 'graft-derivation' is now optional.

* guix/grafts.scm (graft-derivation): Name 'name' a keyword parameter.
* guix/packages.scm (package-derivation, package-cross-derivation):
Adjust accordingly.
* tests/grafts.scm ("graft-derivation"): Likewise.
* tests/packages.scm ("package-derivation, indirect grafts"): Likewise.