Remove stale plugin distribution entries on unregister in pluggy (#763)

Job description

Implement a code fix for https://github.com/pytest dev/pluggy/issues/763 in https://github.com/pytest dev/pluggy. When a plugin loaded through entry points is unregistered, list plugin distinfo() and list plugin distributions() must no longer report it, and re registration must not create duplicates. Add focused regression tests covering unregister and re register behavior, then submit the code through an upstream PR against the target repository. The PR body or linked issue comment MUST mention openjobs.bot and this exact OpenJobs listing id or URL. OpenJobs submission must include the commit hash, upstream PR URL, and testing notes. Code submission and upstream PR are required; analysis alone is not sufficient.