Merge pull request #5908 from cstrahan/hoogle-local-ng

haskell-ng: hoogleLocal support

I believe this code ought to be used as follows:

    {
        packageOverrides = pkgs : let self = pkgs.pkgs; in rec {

        hsEnv = pkgs.haskell-ng.packages.ghc784.ghcWithPackages (p: with p;
                (haskellDev p) 
                ++(myPackages p) 
                # add more packages here
                );
        hsEnvHoogle = withHoogle hsEnv;

        withHoogle = haskellEnv: with pkgs.haskellngPackages;
         import <nixpkgs/pkgs/development/libraries/haskell/hoogle/local.nix> {
          stdenv = pkgs.stdenv;
          inherit hoogle rehoo ghc;
          packages = haskellEnv.paths;
        };
      };
    }