haskell infra: Misc cleanups

 - The haskell lib is very close to not relying on Nixpkgs. I think
   this is good---simpler to think about and matches Nixpkgs's lib.

 - The haskell lib is only imported once

 - stdenv is exposed more shallowly so it can be overriden more easily.
   I'll eventually use this on Darwin to avoid the Sierra shared
   library problems (unless changes are to be made system-wide).

Closes https://github.com/NixOS/nixpkgs/pull/27840.