avoid recursive find

[dirk-thomas]

In the case where selected_rmw_implementation is rmw_implementation it shouldn't call find_package for it since the entire logic already happens in the CMake config file of rmw_implementation so it is already "inside" a find_package(rmw_implementation) call.

This only doesn't end in an infinite recursion due to the custom logic in the generated CMake config files which returns early from repeated finds: https://github.com/ament/ament_cmake/blob/6d171d79584319c216e6880c46372224cd3ae6eb/ament_cmake_core/cmake/core/templates/nameConfig.cmake.in#L3-L7

Connect to ros2/ros2#544.

[dirk-thomas]

CI with 2 RMWs:

• Linux
• Linux-aarch64
• macOS
• Windows

CI with 1 RMW:

• Linux
• Linux-aarch64
• macOS
• Windows

[dirk-thomas]

While this commit (ament/ament_cmake@8003349) was added after CI was triggered the results still hold. Obviously there were no FATAL_ERRORs in the builds so the change is only relevant for overlay builds.

[mikaelarguedas]

lgtm