Chore: Use `snake_case` directory naming style and conform to other filesystem-level aspects of PEP-8

[some-rando-rl]

Right now most directories containing submodules in this project use PascalCase. I believe it would be more idiomatic (and more aligned with PEP-8) if we used a snake_case naming style.

Since this change is very cross-cutting and liable to touch a lot of files, we should also take the opportunity to ensure that any other aspects of this project's file structure align with PEP-8 or other similar accepted PEPs.