grafts: Graft recursively.

grafts: Graft recursively.

Fixes <http://bugs.gnu.org/22139>.

* guix/grafts.scm (graft-derivation): Rename to...
(graft-derivation/shallow): ... this.
(graft-origin-file-name, item->deriver, non-self-references)
(cumulative-grafts, graft-derivation): New procedures
* tests/grafts.scm ("graft-derivation, grafted item is a direct
dependency"): Clarify title.  Use 'grafted' instead of 'graft' to refer
to the grafted derivation.
("graft-derivation, grafted item is an indirect dependency")
("graft-derivation, no dependencies on grafted output"): New tests.
* guix/packages.scm (input-graft): Change to take a package instead of
an input.
(input-cross-graft): Likewise.
(fold-bag-dependencies): New procedure.
(bag-grafts): Rewrite in terms of 'fold-bag-dependencies'.
* tests/packages.scm ("package-derivation, indirect grafts"): Comment out.
* doc/guix.texi (Security Updates): Mention run-time dependencies and
recursive grafting.