[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.

wpilibc/src/main/native/include/frc/controller/PIDController.h

@@ -239,3 +239,9 @@ class PIDController : public frc::Sendable,
 };
 
 }  // namespace frc2
+
+namespace frc {
+
+using frc2::PIDController;
+
+}  // namespace frc