Revisit export configuration

[artivis]

While building ROS 2 from source together with Nav2 and BT I've encountered a couple issues with BT.

• First the Config.cmake file exported was named BehaviorTreeV3Config.cmake whereas, the project being named behaviortree_cpp_v3, one would expect the file to be behaviortree_cpp_v3Config.cmake.
• Second, for some reason I would never enter the if(ament_cmake_FOUND) conditional block, thus the behaviortree_cpp_v3_TARGETS would never be automatically exported by ament and thus never found when BT is included in another project; ament_target_dependencies depends on it to properly set up the interfaces.

This PR adresses both issues by revisiting the project export configuration following the tutorial on CMake website.

Marking as draft since I've only tested my use case and this may break others.

[facontidavide]

Look good to me, but did you check if the NON ROS compilation still work?