* guix/build/graft.scm (rewrite-directory)[rewrite-leaf]: Change to take
a single parameter. Add call to 'lstat'. Factorize result of
'destination'.
Use 'find-files' instead of 'file-system-fold'.
* guix/derivations.scm (graft-derivation): New procedure.
* guix/build/graft.scm: New file.
* Makefile.am (MODULES): Add it.
* tests/derivations.scm ("graft-derivation"): New test.