Restore the listing of Darwin packages.

`nix-env -qaP -A pkgs.darwin`

pkgs.darwin.libutil:
  Commented-out because the package definition doesn't exist. The source
  doesn't even provide a Makefile...

pkgs.darwin.objc4_pure:
  Commented-out because the package is broken and referencing unknown
  applefetchsource and libc_old names. It doesn't seem to be used by any
  other packages too.