Ament exports won't work on cmake 3.24+ since they changed export file generator

[topin89]

In short, this is how error looks on cmake 3.24:

CMake Error at /home/username/workspace/install/nav_2d_msgs/share/nav_2d_msgs/cmake/ament_cmake_export_targets-extras.cmake:24 (message):
  Failed to find exported target names in                                                                                                                                                                         
  '/home/username/workspace/install/nav_2d_msgs/share/nav_2d_msgs/cmake/nav_2d_msgs__rosidl_generator_cExport.cmake'                                                                                
Call Stack (most recent call first):                                                                                                                                                                              
  /home/username/workspace/install/nav_2d_msgs/share/nav_2d_msgs/cmake/nav_2d_msgsConfig.cmake:42 (include)                                                                                         
  CMakeLists.txt:6 (find_package)     

This is relevant part of ament_cmake_export_targets-extras.cmake:

    set(_regex "foreach\\(_expectedTarget (.+)\\)")
    file(
      STRINGS "${_export_file}" _foreach_targets
      REGEX "${_regex}")

This is how relevant part looks like in cmake 3.23.3 (link):

os << "# Protect against multiple inclusion, which would fail when already "
        "imported targets are added once more.\n"
        "set(_targetsDefined)\n"
        "set(_targetsNotDefined)\n"
        "set(_expectedTargets)\n"
        "foreach(_expectedTarget " << expectedTargets << ")\n"
        "  list(APPEND _expectedTargets ${_expectedTarget})\n"
        /* ... */
        "\n\n";

And this is how relevant part looks like in cmake 3.24 (link):

os << "# Protect against multiple inclusion, which would fail when already "
        "imported targets are added once more.\n"
        "set(_cmake_targets_defined \"\")\n"
        "set(_cmake_targets_not_defined \"\")\n"
        "set(_cmake_expected_targets \"\")\n"
        "foreach(_cmake_expected_target IN ITEMS " << expectedTargets << ")\n"
        "  list(APPEND _cmake_expected_targets \"${_cmake_expected_target}\")\n"
        /* ... */
        "\n\n";

This kind of situation was predicted about 9 month ago, along with a solution, so at least there is a known way to fix it. Alternative quick fix is to use regex like foreach\\((?:_expectedTarget|_cmake_expected_target) (.+)\\), less robust but faster to implement and it has a backward compatibility with previously generated packages. Still, probably better first check if IMPORTED_TARGETS exists, and if not, check for old regex.