gap: add packageSet option

Two reasons for this:

- more fine-grained space/functionality tradeoff

- preparation for the sage 8.6 update, which finally doesn't need a
  downgraded gap anymore but does break when unexpected (non-standard)
  packages are installed. Details: https://trac.sagemath.org/ticket/26983

The proper way to deal with gap packages would be to create a package
set, package each one individually and have something like gap
equivalent of `python.withPackages`. I am not willing to do that
however.