* guix/packages.scm (bag->derivation): Turn into a monadic procedure by
remove 'store' parameter and removing the call to 'store-lower'.
(bag->cross-derivation): Likewise.
(bag->derivation*): New procedure.
(package-derivation, package-cross-derivation): Use it instead of
'bag->derivation'.
* tests/packages.scm ("bag->derivation"): Change to monadic style.
("bag->derivation, cross-compilation"): Likewise.
