Add interface for target selector

This adds an interface to provide something to choose a target to guide
into based on the current robot state.

This does not actually add a real implementation for target choosing.

Change-Id: I73bfde640dbe0851847f9f77710614607bad96ea
diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index 7dedc71..c99053f 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -71,6 +71,7 @@
     deps = [
         ":drivetrain_config",
         ":hybrid_ekf",
+        "//frc971/control_loops:pose",
     ],
 )
 
@@ -111,6 +112,7 @@
     deps = [
         ":drivetrain_config",
         ":drivetrain_queue",
+        ":localizer",
         "//aos:math",
         "//aos/util:math",
         "//frc971/control_loops:c2d",