haskell-modules: refactor package set

This change is effectively a no-op to nixpkgs. However, it gives users
the flexibility to create and maintain their own package sets per
project, while benefiting from nix's Haskell configurations.

I would make immediate use of this change in stack2nix, a project that
generates nix expressions for all the dependencies of a given Haskell
project. @domenkozar is familiar with the motivations and helped
refine this change