fixtures: --fixtures/--fixtures-per-test don't go through pytest_collection

These two modes currently use session.perform_collect() directly to perform the collection.

I don't see a reason for them to bypass the regular collection hook. If plugins do things with the hook on a regular run, they should do it for these two modes as well, just as they do with --collect-only.

This could possibly cause some breakage (though unlikely I think), so let's schedule this for pytest 10.0.