Python code generation: wrong import

Hey,

I've put together a small repro case: https://github.com/Tinche/fb-python-issue

Check out the repo and run flatc -p -I . **/*.fbs.

I'm running flatc version 1.9.0 (Apr 6 2018 01:08:33) on macOs.

This is a minimized case from what we're using at work. This line in particular is the problem: https://github.com/Tinche/fb-python-issue/blob/master/fbs/networking/socket/inbox/GetConversationsResponse.py#L28

(It should read: from ....inbox.Conversation import Conversation, if we're favoring relative imports.)

The code generator probably gets confused by the top level packages both being named 'inbox'.