We had a few "overrides" in configuration-common.nix that were really
extensions. They introduced packages that weren't in hackage-packages.nix.
The advantage of having a dedicated file for these packages is that we can still
place Nix-specific overrides to these packages in configuration-nix.nix. We
weren't able do this before because configuration-nix.nix extended only the
packages from hackage-packages.nix.
