Please, be able to generate the Nixpkgs `haskellPackages` set

[Anton-Latukha]

After HNix would be able to evaluate its own closure and build itself (#621)

The Nix is not a typesafe language, Nixpkgs should have a number of internal type inconsistencies. Type inconsistencies should be solved for strictly typed language, so strictly speaking types should be solved in Nixpkgs before HNix would be able to propagate into the code.

• There should be a start somewhere
• The start should be most effective.
• One of the most self-consistent and mostly free of type errors domains of the Nixpgks is the haskellPackages.
  • It is a builds of Haskell projects that are initially kept by the Haskell tooling ecosystem and the majority of the domain is autogenerated through Haskell tooling from Hackage, that is why it is the most consistent part.
• Haskell domain in Nix is the most important and close domain for us.
• If work with haskellPackages would be achieved in HNix - that would create a ground and growing and direction to the community, so the community would grow, and so chipping away at the front would happen.
• Since the community is connected to Nixpkgs & NixOS - a community around haskellPackages would propagate solving type inconsistencies in Nixpkgs.

So the best way to start is from the most type safe domains of Nixpkgs, one of such domains is haskellPackages and it is the main goal.