This reverts commit dfb0f254844150d05d34150a975e6479b883d409, reversing
changes made to 7f8ff024372f985e96b438fa5591d9b43fc6e7bd. 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)