CLI: support paths to URDF files (i.e. without ament_index_python ROS package registry)

[mhubii]

1. The URDFParser factory inside the CLI:

   roboreg/cli/rr_hydra.py

   Line 165 in 64728da

   urdf_parser = URDFParser.from_ros_xacro(

   internally relies on ament_index_python:

   roboreg/roboreg/io/parsers.py

   Line 64 in 64728da

   from ament_index_python import get_package_share_directory

   to turn xacro into URDF.

2. The Robot factory:

   roboreg/cli/rr_hydra.py

   Line 187 in 64728da

   robot = Robot.from_urdf_parser(

   internally uses the ament_index_python:

   roboreg/roboreg/io/parsers.py

   Line 161 in 64728da

   from ament_index_python import get_package_share_directory

   to compile the mesh paths.