Some cleanup in package.xml and CMakeLists.txt

- builds broke sporadically (I think because of the missing *_gencpp in
   add_dependencies) with missing Packet.h file.
 - I’m no catkin expert, but these changes make catkin_lint happy (no
   more errors at least).
Must have been copy and paste bug, I don't see any history of a `plugin_description.xml` file.