Adam/generalized draft import export 2

@dmitchell -- is this the right idea?

https://openedx.atlassian.net/browse/TNL-574
Hm. With old exports, only draft verticals would have this attribute. All of them would be returned by `get_roots_from_node_list`. @dmitchell , can you imagine a scenario where `index_in_children_list` wouldn't be in `xml_attributes`? Why did you add the `if any` to the doc string?