Split up COMPAS FAB backends

To ease the create of new backends and simplify deployment of COMPAS FAB in general, the backends should be split into completely new packges: `compas_fab_ros`, `compas_fab_pybullet` and `compas_fab_vrep` (or `compas_fab_coppeliasim` if we update it to the latest version).

All pure-python implementations of backend features (i.e. if we add our own planner implementation at some point), should stay in this package. All implementations that require out-of-process tools to run should become its own package. The pybullet is on the verge of being ok to stay so it might be discussed.

Conda packages will also need to be split, and ideally also a combined `compas_fab_backend_bundle` could be created to install all backends at once.