Split up zeroing library BUILD rules

There was no good reason for one build rule to have both the interface
and a bunch of implementations; split it up and make the relevant users
depend on the libraries directly.

Change-Id: If6a0aa49dd7ea588b7325604658bc0594db4aac0
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/y2020/constants.h b/y2020/constants.h
index 83b2cec..7332b4f 100644
--- a/y2020/constants.h
+++ b/y2020/constants.h
@@ -8,6 +8,9 @@
 #include "frc971/constants.h"
 #include "frc971/control_loops/static_zeroing_single_dof_profiled_subsystem.h"
 #include "frc971/shooter_interpolation/interpolation.h"
+#include "frc971/zeroing/absolute_and_absolute_encoder.h"
+#include "frc971/zeroing/absolute_encoder.h"
+#include "frc971/zeroing/pot_and_absolute_encoder.h"
 #include "y2020/control_loops/drivetrain/drivetrain_dog_motor_plant.h"
 #include "y2020/control_loops/superstructure/accelerator/accelerator_plant.h"
 #include "y2020/control_loops/superstructure/control_panel/control_panel_plant.h"