hackage2nix: filter Hackage tree with preferred-versions file
hackage2nix: filter Hackage tree with preferred-versions file

We consider only packages and/or package versions that satisfy the
constraints in the preferred-versions file. This means that we won't
distribute deprecated packages by default.

Resolves https://github.com/NixOS/cabal2nix/issues/128.
Resolves https://github.com/NixOS/cabal2nix/issues/185.
Resolves https://github.com/NixOS/nixpkgs/issues/8924.

psibi/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix,gridaphobe/cabal2nix