[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/test/native/include/MockMotorController.h

@@ -4,11 +4,11 @@
 
 #pragma once
 
-#include "frc/SpeedController.h"
+#include "frc/motorcontrol/MotorController.h"
 
 namespace frc {
 
-class MockSpeedController : public SpeedController {
+class MockMotorController : public MotorController {
  public:
   void Set(double speed) override;
   double Get() const override;