Add some debugging logs to `self_outdated_check`
Add some debugging logs to `self_outdated_check`

This would make it easier to diagnose what an issue might be, related to
this logic, since the things that affect whether the message is printed,
are now available in the debugging logs.

pradyunsg/pip,pradyunsg/pip,sbidoul/pip,pypa/pip,sbidoul/pip,pfmoore/pip,pypa/pip,pfmoore/pip