Internal category for "assets" or "accessories"

[stephane-caron]

#72 by @kevinzakka adds a model of the realsense d435i, which is a robot accessory rather than a robot description in itself. It also happens that URDF descriptions depend on external packages (i.e. contain package://not_the_description_itself URIs).

Ideas on how we could handle accessories in robot_descriptions.py:

1. Add them as robot descriptions
2. Expand robot descriptions to allow listing accessories, which would be in effect dependencies.
   • One pro: that way accessories can be added automatically to the package dirs, which in turn can help projects resolve package:// URIs that were not resolved before.