Replace custom "set" library by Skylib's sets.bzl

[aherrmann]

rules_haskell contains a home-grown set library in haskell/private/set.bzl. However, it's implementation is very similar to the one provided by Skylib. We should switch to the Skylib provided one and remove the homegrown version.

[evertedsphere]

Skylib's version doesn't expose mutable_{union,difference}, so I've opened bazelbuild/bazel-skylib#415 for that.