Remove stale plugin distribution entries on unregister

[antiren1980-ship-it]

Summary

load_setuptools_entrypoints() appends to _plugin_distinfo, but unregister() only removed the plugin from _name2plugin and its hook callers. As a result:

• list_plugin_distinfo() and list_plugin_distributions() kept reporting an unregistered plugin;
• re-registering the plugin added a duplicate entry.

unregister() now also drops the plugin's _plugin_distinfo entries, so both views stay in sync and re-registration does not create duplicates.

Fix

src/pluggy/_manager.py — after removing the plugin from _name2plugin, filter it out of _plugin_distinfo:

self._plugin_distinfo[:] = [
    (p, dist) for (p, dist) in self._plugin_distinfo if p is not plugin
]

Tests

Added test_unregister_removes_plugin_distinfo which:

1. loads a plugin via load_setuptools_entrypoints(),
2. unregisters it and asserts both list_plugin_distinfo() and list_plugin_distributions() are empty,
3. re-registers it and asserts there is exactly one entry (no duplicate).

The test fails on main and passes with the fix.

78 passed

Fixes #763