Claiming that a hackage package is available in nixpkgs when we
explicitly mark it as broken is less helpful, so let's filter them out.
This changes the number of reported packages from 17137 to 10937.
Fixes#243512
Since this list is intended for Hackage, we need to make sure that only
packages with versions conforming to the PVP versioning scheme are
actually included in the CSV or Hackage will reject it.
