[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/PIDOutput.h

@@ -0,0 +1,22 @@
+// 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/Base.h"
+
+namespace frc {
+
+/**
+ * PIDOutput interface is a generic output for the PID class.
+ *
+ * PWMs use this class. Users implement this interface to allow for a
+ * PIDController to read directly from the inputs.
+ */
+class PIDOutput {
+ public:
+  virtual void PIDWrite(double output) = 0;
+};
+
+}  // namespace frc