nix: poetry2nix -> buildPythonPackage

Enables users to use NXC without rebuilding a full Python stack.

Con: nix shell and poetry shell no longer provide the same Python stack, but I think this is worth it as Nix is more powerful.

I tested the packaging of this with OAR on docker, g5k-image and g5k-ramdisk flavors. I could not detect any problem.