Split beat.core and beat.backend.python
Following a discussion with @andre.anjos , few beat.core functionalities need to be in beat.backend.python.
To achieve this, we need to:
- Move all the relevant code from beat.core to beat.backend.python
- Make beat.core depends on beat.backend.python
- Make all packages use features from beat.backend.python (when they were moved from beat.core to beat.backend.python)