Adding overrides is an uphill battle we are never going to win and
poetry2nix should try to attempt to make things "just work" to the
largest extent possible.
This only increases the build closures slightly while making
poetry2nix much easier to use.
setuptools_scm itself does not have any dependencies so will not cause
compatibility issues.
- Extract utility functions to ./lib.nix
- Extract buildPythonPackage to ./mk-poetry-dep.nix
- Replace some functions with existing builtin functions
- Use lib.partition for python package splitting
- Drop package filtering on the top-level and leave it to pep425.nix