[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

@@ -0,0 +1,25 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include "frc/motorcontrol/MotorController.h"
+
+namespace frc {
+
+class MockMotorController : public MotorController {
+ public:
+  void Set(double speed) override;
+  double Get() const override;
+  void SetInverted(bool isInverted) override;
+  bool GetInverted() const override;
+  void Disable() override;
+  void StopMotor() override;
+
+ private:
+  double m_speed = 0.0;
+  bool m_isInverted = false;
+};
+
+}  // namespace frc