Users without opensplice installed cannot use the fat archive to build overlays if the archive was built with opensplice support.

If opensplice support is built in the underlay,

1. install/share/rosidl_default_generators/cmake/rosidl_default_generators-extras.cmake lists opensplice typesupport as a default generator.
2. rmw_opensplice_cpp will be registered in the ament index as a supported rmw implementation, so call_for_each_rmw_implementation will find it.

(1) causes issues because opensplice is required in this findpackage call

(2) causes issues because rosidl_generator_py_get_typesupports.cmake calls call_for_each_rmw_implementation which find_packages each rmw_implementation. There might be other side effects.

Allowing rmw_opensplice_cpp-extras.cmake to give a warning when it can't find opensplice, instead of fail, should address both points. If there are other points that should be considered please list them here