This command was useful when NixOS was spread across multiple
repositories, but now it's pretty pointless (and obfuscates what
happens, i.e. "git clone git://github.com/NixOS/nixpkgs.git").
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
