This message is confusing in uv sync, because... --clear isn't provided, that's jut the default uv sync behavior.

This may require adding an optional Option<OnExistingSource> enum to the OnExisting type so we can differentiate between OnExisting::Remove(Some(OnExistingSource::ClearFlag)) and OnExisting::Remove(None). We'd also be able to differentiate between OneExistingSource::ClearEnv if we wanted? I'm not sure if this is the best approach, just an option.