ament_index_python not being registered in the package ament_index resource

[ruffsl]

It seems from inspecting the nightly fat archive, that ament_index_python is not being included in the ament_index resource or listing packages. cc @dirk-thomas

``` $ ls share/ament_index/resource_index/packages/ | grep ament ament_cmake ament_cmake_auto ament_cmake_clang_format ament_cmake_clang_tidy ament_cmake_copyright ament_cmake_core ament_cmake_cppcheck ament_cmake_cpplint ament_cmake_export_definitions ament_cmake_export_dependencies ament_cmake_export_include_directories ament_cmake_export_interfaces ament_cmake_export_libraries ament_cmake_export_link_flags ament_cmake_flake8 ament_cmake_gmock ament_cmake_gtest ament_cmake_include_directories ament_cmake_libraries ament_cmake_lint_cmake ament_cmake_mypy ament_cmake_nose ament_cmake_pclint ament_cmake_pep257 ament_cmake_pep8 ament_cmake_pyflakes ament_cmake_pytest ament_cmake_python ament_cmake_ros ament_cmake_target_dependencies ament_cmake_test ament_cmake_uncrustify ament_cmake_xmllint ament_index_cpp ament_lint_auto ament_lint_common launch_testing_ament_cmake ```

[dirk-thomas]

This is a known regression and pending review will get fixed by colcon/colcon-ros#79.