Add multi-file model output

[PaleNeutron]

Closes #88.

This adds a CLI option for writing generated models into a directory, with one Python file per generated model:

• --output-directory <dir> creates the directory if needed
• shared declarations are written to base_model.py
• an empty __init__.py is created
• each generated model is written to its own .py file

The generator API now also accepts generate(multi_file=True) and returns a mapping of module names to generated source strings. Calling generate() without arguments preserves the existing single-file behavior.

Verification:

• uv run pytest
• uv run ruff check .

[coveralls]

coverage: 97.802% (-0.04%) from 97.839% — PaleNeutron:feature/multi-file-output-pr into agronholm:master

[sheinbergon]

@PaleNeutron thank you for your contribution. While there has been a long discussion about this request in the original issue, I feel the a file per module maybe a bit of an exaggeration. I would say a file per schema makes more sense, but feel free to counter that idea. @agronholm wdyt?

[PaleNeutron]

@sheinbergon , we subscribe financial data into a database and the provider put about 2000 tables in one schema. Split models by schema can not solve this problem.

BTW, generally schema count is less than 10 and generate each one in a loop is acceptable, no need to create a new option.

[sheinbergon]

One smal change request

[sheinbergon]

@sheinbergon , we subscribe financial data into a database and the provider put about 2000 tables in one schema. Split models by schema can not solve this problem.

BTW, generally schema count is less than 10 and generate each one in a loop is acceptable, no need to create a new option.

Alright, Fine by me. Submitted one small change request