[wpilib] Remove PIDController, PIDOutput, PIDSource

Move them to the old commands vendordep so that PIDCommand and PIDSubsystem
continue to work.

This also removes Filter and LinearDigitalFilter.

wpilibOldCommands/src/main/native/include/frc/PIDInterface.h

@@ -0,0 +1,34 @@
+// 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 <wpi/deprecated.h>
+
+namespace frc {
+
+/**
+ * Interface for PID Control Loop.
+ *
+ * @deprecated All APIs which use this have been deprecated.
+ */
+class PIDInterface {
+ public:
+  WPI_DEPRECATED("All APIs which use this have been deprecated.")
+  PIDInterface() = default;
+  PIDInterface(PIDInterface&&) = default;
+  PIDInterface& operator=(PIDInterface&&) = default;
+
+  virtual void SetPID(double p, double i, double d) = 0;
+  virtual double GetP() const = 0;
+  virtual double GetI() const = 0;
+  virtual double GetD() const = 0;
+
+  virtual void SetSetpoint(double setpoint) = 0;
+  virtual double GetSetpoint() const = 0;
+
+  virtual void Reset() = 0;
+};
+
+}  // namespace frc