Expose a pytest_plugin module so other loaders can reuse for tests
This does not expose the pytest-plugin through the setuptools entry as this can be problematic. Instead that relies on the specific modules (loader-*) declaring they need the dependency in their root conftest.
As i'm heading there when i'm done with the check-snapshot improvments (and tagging a new release). Might as well prepare some cleanup as well.
Related to T2484
Test Plan
tox
Migrated from D3495 (view on Phabricator)