[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/java/edu/wpi/first/wpilibj/PIDOutput.java

@@ -0,0 +1,21 @@
+// 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.
+
+package edu.wpi.first.wpilibj;
+
+/**
+ * This interface allows PIDController to write it's results to its output.
+ *
+ * @deprecated Use DoubleConsumer and new PIDController class.
+ */
+@FunctionalInterface
+@Deprecated(since = "2020", forRemoval = true)
+public interface PIDOutput {
+  /**
+   * Set the output to the value calculated by PIDController.
+   *
+   * @param output the value calculated by PIDController
+   */
+  void pidWrite(double output);
+}