missing submodules in install

[alxsimon]

While trying to insall sgkit in Guix I had an error with the three submodules not being found.

This can be fixed by specifying the submodules in the pyproject.toml here https://github.com/sgkit-dev/sgkit/blob/af2b62378548b4b25a2937c21bbe95d95948cfea/pyproject.toml#L60C1-L60C21

packages = ["sgkit", "sgkit.stats", "sgkit.distance", "sgkit.io"]

Not sure if it's packaging tooling specific, but other install systems might have issues with that too.