1) change `get_installed_distributions` so that it has a different OR check when both `local_only` *and* `user_only` are provided.
2) throw a commanderror when someone tries to pass --user and `location.virtualenv_no_global()` is true.  i.e. when you're in a no-global virtualenv, you have no visibility to the user site.  Or better, don't even add the `--user` option when that's true.