Keep `nix/haskell.nix inlined in `flake`

[mikekeke]

Issue initiated by this comment from @ngua:

This (i.e. nix/haskell.nix) appears to be a new module, but it's not being imported anywhere AFAICT. I'd honestly recommend just keeping it inline in the flake anyway though, it's not a lot of code and I'm not sure if there's a significant benefit having it here in a separate module

[mikekeke]

Closed, see #73 (comment)