[wpilib] Move motor controllers to motorcontrol package (#3302)

Also deprecate SpeedController in favor of motorcontrol.MotorController and
SpeedControllerGroup in favor of motorcontrol.MotorControllerGroup.

The MotorController interface is derived from the SpeedController interface
so that code such as SpeedController x = new VictorSP(1) continues to
compile (just with a warning).

SpeedControllerGroup and MotorControllerGroup are independent classes;
both implement the MotorController interface.

wpilibcExamples/src/main/cpp/examples/TankDriveXboxController/cpp/Robot.cpp

@@ -3,10 +3,10 @@
 // the WPILib BSD license file in the root directory of this project.
 
 #include <frc/GenericHID.h>
-#include <frc/PWMSparkMax.h>
 #include <frc/TimedRobot.h>
 #include <frc/XboxController.h>
 #include <frc/drive/DifferentialDrive.h>
+#include <frc/motorcontrol/PWMSparkMax.h>
 
 /**
  * This is a demo program showing the use of the DifferentialDrive class.