Pytest project structure not supported by Pyrefly import root heuristics

Pytest projects typically have two checked directories:
- `src/`
- `src/`'s import root is itself. All files in `src/` import from `src/`
- `tests/` (maybe `test/`?)
- `tests/` import root is its parent directory. We should support this in Configs, since right now the default config won't work at all in this directory.

We should do one of:
- get rid of import root (or just use it for debugging stuff) and move its information into `search_path`
- make import root an enum with a variant that can possibly work with multiple roots (one primary and one secondary for tests)