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/y2019/constants.cc b/y2019/constants.cc
index 10582ed..380899a 100644
--- a/y2019/constants.cc
+++ b/y2019/constants.cc
@@ -12,6 +12,8 @@
 
 #include "aos/network/team_number.h"
 #include "aos/stl_mutex/stl_mutex.h"
+#include "frc971/zeroing/absolute_encoder.h"
+#include "frc971/zeroing/pot_and_absolute_encoder.h"
 #include "y2019/control_loops/superstructure/elevator/integral_elevator_plant.h"
 #include "y2019/control_loops/superstructure/intake/integral_intake_plant.h"
 #include "y2019/control_loops/superstructure/stilts/integral_stilts_plant.h"