Ignore non-useful files in hpio.nix

[dhess]

nix-build does not settle on a fixed point; repeated nix-builds with no changes will run each time rather than returning a cached result.

I believe this is because the hpio.nix generated by cabal2nix is using the top-level directory, so things like result files are factoring into the generated hash.

Figure out a way to get around this, perhaps by putting the sdist files in a subdirectory of the top-level directory.

[dhess]

See:

http://hackage.haskell.org/package/hspec-checkers-0.1.0/src/default.nix
NixOS/cabal2nix#112

Probably want to apply builtins.filterSource in the haskell-overrides.nix overlay, when we callPackage on the various hpio Nix files.

[dhess]

pkgs.lib.cleanSource and pkgs.lib.cleanSourceWith are what we need here.