Revert "Merge pull request #27209 from nc6/nc/haskell-multiple-out"

This reverts commit dfb0f254, reversing
changes made to 7f8ff024. These changes broke
the ghcWithPackages wrapper:

    nix-shell -p "haskellPackages.ghcWithPackages (ps: [ps.mtl])" --run "ghc-pkg list mtl"
    /nix/store/szz84j5k1dy3jdashis6ws28d8l8zxxb-ghc-8.0.2-with-packages/lib/ghc-8.0.2/package.conf.d
        (no packages)