Fix return value from CMAKE_MODULE_PATH hook (#147)

This was a regression introduced by
5704d83 which was hidden by some
nuanced API characteristics for environment hook extensions, which would
manifest as silently missing environment hooks for CMAKE_MODULE_PATH.

colcon_cmake/environment/cmake_module_path.py

@@ -44,7 +44,7 @@ def create_environment_hooks(self, prefix_path, pkg_name):  # noqa: D102
                         str(path.relative_to(prefix_path)),
                         mode='prepend')
 
-        return hooks.values()
+        return [hook for hook_list in hooks.values() for hook in hook_list]
 
     def _get_potential_cmake_module_paths(self, prefix_path, pkg_name):
         paths = []