Instead of discovering unit tests using loader.discover() by passing
it a set of starting and top level directories, we now discover
unit tests by loading them from specific test module objects. This
makes it easier to include and exclude specific bots from testing.
In order to keep all three packages (zulip, zulip_bots,
zulip_botserver) in the same repo, all package files must now
be nested one level deeper.
For instance, python-zulip-api/zulip_bots/zulip_bots/bots/, instead
of python-zulip-api/zulip_bots/bots/.
tools/server_lib contains files copied as-is, or with minor
modifications, from the zulip/zulip repository when the api
code was split into this separate repository.
