* guix/derivations.scm (derivation): Add `dependency-graphs' keyword
parameter; honor it.
* tests/derivations.scm (bootstrap-binary): New procedure.
(%bash): Use it.
(%mkdir): New variable.
(directory-contents): Add `slurp' optional parameter.
("derivation with #:dependency-graphs"): New test.
* doc/guix.texi (Derivations): Update accordingly.