derivations: Compile the #:modules passed to `build-expression->derivation'.

* guix/derivations.scm (imported-files)[parent-dirs]: Move to...
  (parent-directories): ... here.  New procedure.
  (compiled-modules): New procedure.
  (build-expression->derivation): Use it.

* tests/derivations.scm ("build-expression->derivation with modules"):
  New test.