Python-packages from flakes

Wheels on pypi are great,
but at times you need to build 'complicated' python packages from source,
and in a way that exceeds a 'stick it into mach-nix buildPythonPackage'.

An example for this is rust/pyo3/maturin based packages.

This update allows you to specifiy package = {method="useFlake", flakeName="xyz"},
and then use a [flake.xyz] section.

The flake must export a mach-nix-build-python-package function - see https://github.com/IMTMarburg/mbf_bam/blob/default/flake.nix for an example.