[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.

wpilibc/src/main/native/include/frc/SpeedController.h

@@ -5,13 +5,14 @@
 #pragma once
 
 #include <units/voltage.h>
+#include <wpi/deprecated.h>
 
 namespace frc {
 
 /**
  * Interface for speed controlling devices.
  */
-class SpeedController {
+class WPI_DEPRECATED("use MotorController") SpeedController {
  public:
   virtual ~SpeedController() = default;