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/.
Instead of using the `scripts` keyword, we now use the
`console_scripts` entry point to point to the zulip-send script
to be installed. This is what the Python Packaging User Guide
recommends for better cross-platform compatibility.