Remove cache files created for namespace packages

[bzoracler]

This PR fixes an issue where cache files created from namespace packages are not cleaned up.

---

For the following concrete example,

- case: my_test_case
  main: |
    import pkg.module
    import nspkg.module
  files:
    - path: pkg/__init__.py
    - path: pkg/module.py
    - path: nspkg/module.py

during pytest, mypy creates the following under /tmp/.mypy_cache/3.1x/:

• pkg/__init__.{data & meta}.json
• pkg/module.{data & meta}.json
• nspkg/module.{data & meta}.json
• nspkg.{data & meta}.json

The existing implementation of YamlTestItem.remove_cache_files only passes pathlib.Path arguments pointing to files, not directories, so while mypy creates .{data,meta}.json cache files for namespace packages' directory names, .remove_cache_files() never removes these cache files.

[sobolevn]

Thanks! Can we test?

[bzoracler]

This guards against removal of unrelated cache files.

For example, if we used pytest-mypy-plugins to type-check a file called abc-stubs.pyi, it should not remove the cache files abc.data.json and abc.meta.json.

I didn't add tests for this; testing this requires getting pytest-mypy-plugins to type-check a module that's not called main.py, or to arbitrarily check an entire package, and I don't think we can do that?