[RFC] make the public API stricter

[jameslamb]

Description

Since this library is still in development and breaking changes are not too disruptive, I think it's worth making the public API stricter right now.

Through some mix of the following:

• requiring keyword-only arguments wherever possible
• using __all__ entries in modules to limit what is exported (affects * imports and things like sphinx autodoc)
• prefixing private functions, classes, methods, constants, etc. with _

Benefits of this work

Clarifies the boundary between the private and public API for users of the library.

Expands the set of implementation details that could be changed in the future without worrying about breaking users.

Reduces the risk of naming conflicts wherever * imports are used.

Acceptance Criteria

• every non-testing module in the Python package has an __all__ list containing only its public attributes
• keyword-only arguments are required wherever possible
• every object that is not intended to be public is prefixed with a _

Approach

See these related issues for reference:

• https://github.com/rapidsai/dependency-file-generator/issues/76
• https://github.com/dmlc/xgboost/issues/6172
• https://github.com/microsoft/LightGBM/pull/5655

Make changes in this project to implement these things.

Notes

N/A

[RAMitchell]

I missed this somehow, but this definitely a great idea.