[WIP] Hackage revisions support

[qrilka]

This PR adds proper Hackage revision support in a way that they are specified for Stack projects (i.e. in stack.yaml and in Stackage snapshots), prior to this PR Nix builds done with derivations from stack2nix did not work exactly the same as Stack builds: revisions in Nix were not the same as the ones used by Stack. This is a WIP as it depends on https://github.com/NixOS/cabal2nix/pull/393 which in its turn depends on https://github.com/peti/hackage-db/pull/9