haskellSrc2nix

Since [1], cabal2nix has been able to parse full platform triples. We
need this for when the system doesn't say enough info (e.g. android).

[1]: https://github.com/NixOS/cabal2nix/commit/0bb88f0009710e013564f24c195fd037e766310e#diff-d9172aeec4039eef8cfcc8a2ab6c0677R143