Remove stale plugin distinfo entries on unregister (#763)

[ciperx]

Summary

Fixes #763.

load_setuptools_entrypoints() appends (plugin, dist) to _plugin_distinfo, but unregister() never removed those entries. After unregistering an entry-point plugin, list_plugin_distinfo() and list_plugin_distributions() still reported it (and that list feeds pytest's plugins: header), and re-registering through entry points appended a duplicate.

unregister() now drops the plugin's entries from _plugin_distinfo (mutating in place, since the attribute is Final).

Changes

• src/pluggy/_manager.py: unregister() removes the plugin from _plugin_distinfo.
• testing/test_pluginmanager.py: regression test — after unregister(), both listing methods return empty, and re-registering through entry points yields exactly one entry.

Testing

python -m pytest testing/test_pluginmanager.py -q -k "setuptools or unregister_removes"
4 passed

python -m pytest testing/ -q
220 passed

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• Issue: list_plugin_distinfo() keeps unregistered plugins #763