benchmarks → plant Relation

File in drake/multibody/benchmarks	Includes file in drake/multibody/plant
acrobot / make_acrobot_plant.h	multibody_plant.h
inclined_plane / inclined_plane_plant.h	multibody_plant.h
kuka_iiwa_robot / drake_kuka_iiwa_robot.h	multibody_plant.h
pendulum / make_pendulum_plant.h	multibody_plant.h