use ament_python_install_package when package is not installed

rosidl_generator_py/cmake/rosidl_generator_py_generate_interfaces.cmake

@@ -121,7 +121,23 @@ rosidl_write_generator_arguments(
 )
 
 if(NOT rosidl_generate_interfaces_SKIP_INSTALL)
-  ament_python_install_package(${PROJECT_NAME} PACKAGE_DIR "${_output_path}")
+  if(${PROJECT_NAME} IN_LIST AMENT_CMAKE_PYTHON_INSTALL_INSTALLED_NAMES)
+    ament_python_install_module("${_output_path}/__init__.py"
+      DESTINATION_SUFFIX "${PROJECT_NAME}"
+    )
+
+    # TODO(esteve): replace this with ament_python_install_module and allow a list
+    # of modules to be passed instead of iterating over _generated_py_files
+    # See https://github.com/ros2/rosidl/issues/89
+    foreach(_generated_py_dir ${_generated_py_dirs})
+      install(DIRECTORY "${_output_path}/${_generated_py_dir}/"
+        DESTINATION "${PYTHON_INSTALL_DIR}/${PROJECT_NAME}/${_generated_py_dir}"
+        PATTERN "*.py"
+      )
+    endforeach()
+  else()
+    ament_python_install_package(${PROJECT_NAME} PACKAGE_DIR "${_output_path}")
+  endif()
 endif()
 
 set(_target_suffix "__py")