The problem is that there is no namespace_packages defined in setup(...) call in setup.py.
It is easy to fix.

How to reproduse:
install style-analyzer: pip3 install style-analyzer and run

>>> import lookout.core
>>> import lookout.style
>>> import lookout.sdk
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'lookout.sdk'

but lookout-sdk, as well as lookout-sdk-ml, are installed as dependencies.