Allow to import code from subdirectory of `contrib_bots/lib`.
Because sometimes bot requires grouping in their directory,
especially for the bot that has configuartion file and documentation.
This allows bots to be stateful. It doesn't handle persistence
after the bot shuts down, but it does store state between
invocations of handle_message.
This commit also starts to build out the infrastructure for
helping Zulip contributors to more easily author bots in
a way that sets up for running some bots on the server itself.
Rafid Aslam
reallyly
Anders Kaseorg
Steve Howell