All the necessary "glue" logic to convert ROS Data to a form that the
DDS/RTPS back-end can use is implemented in packages.
The ROS Node discovery information is statically generated in packages
(similar to the RTPS Participant Data), and a special dds writer is
implemented (ros_static_discovery_writer) that has this static data as
its only payload sample.
Some definitions are moved out of rtps_config_package to prevent
circular package dependency.
