stack: handle external-children just like other states This also fixes color labels: previously when external-children was combined with another state, --color=debug would show something like: [topic.stack.index topic.stack.index.current - external-children|s4] Now there is a separate label for each state.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)