This reinstates pre-gexp behavior where 'expand-input' would explicitly
pass #:graft? #f in recursive calls, thereby preventing redundant calls
to 'bag-grafts'.
* guix/packages.scm (expand-input): Turn into a monadic procedure.
Lower INPUT when it's a package, passing #:graft? #f.
(bag->derivation, bag->cross-derivation): Adjust accordingly.
* tests/packages.scm ("search paths"): Adjust so BUILD aborts only when
passed the package of interest.
