Why doesn't move_base depend on move_base_msgs

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Looking at the CMakeLists.txt for move_base (link), why is there no mention of move_base_msgs ? I would expect to see something like:

`add_dependencies(move_base move_base_msgs_gencpp)`

and/or see move_base_msgs listed under find_package COMPONENTS

Without that, how is it able to include the following (automatically generated) header in move_base.h:

#include <move_base_msgs/MoveBaseAction.h>

And how does catkin know to generate the MoveBase.action msgs and headers before building move_base ?

Update: A colleague figured out that it's related to whether the packages are built in isolation or not.