The zulip and zulip_botserver packages specify mock as a runtime
dependency, which is only needed for testing during development.
So, it made more sense to move it to requirements.txt so that
it can be installed by ./tools/provision during development.
As the first unittest, this creates a test directory and
abnd adds it tho the excluded pip package files.
There are two `tests` directories now, one in zulip_botserver and one in
zulip. This confuses the unittest runner, leading to failed test imports.
Therefore, we need to tell the package importer that there are multiple
tests directories, all of which should be considered for a search.
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/.
