Since superflore doesn't know when its generating a recipe that it will
be for a buildtool, it can't know that what's in ROS_EXEC_DEPEND are also
buildtools.

  * Make it conditional on ARCH. Otherwise needs to be
    reworked to avoid getting the SOABI from python3-native.

  * See:
    https://github.com/lgsvl/meta-ros2/commit/4b6bb786336695cd04144ff4eeb2e4d6b6b9ad3a
    https://github.com/ros2/rosidl_python/blob/master/python_cmake_module/cmake/Modules/FindPythonExtra.cmake
    https://discourse.ros.org/t/import-error-no-module-named-rclpy-rclpy-cross-compiling/2463/

  * With the patch ros2 cli works well for all listed ARCH-s:

    root@qemuarm64:~# ros2 topic list
    /parameter_events
    /rosout

     root@qemuarm64:~# ros2 topic pub /chatter std_msgs/String "data: Hello world"
     publisher: beginning loop
     publishing #1: std_msgs.msg.String(data='Hello world')

     publishing #2: std_msgs.msg.String(data='Hello world')

     ...

     root@qemuarm64:~# ros2 topic echo /chatter
     data: Hello world

     data: Hello world

     ...

S is set correctly in the generated recipes, so drop setting it in the
bbclass-s.

ros*_distro.bbclass: Remove rviz/rviz2 from DEPENDS & RDEPENDS unless "ros-rviz" is in DISTRO_FEATURES

Previously was in catkin.bbclass .

Also ensure PYTHON_DIR is set when only inherit-ing catkin_ros_prefix.bbclass