Support Python 3.8 importlib.metadata, declare dependency (#229)

The importlib_metadata package is a backport of the importlib.metadata
module from Python 3.8. We should prefer the one coming from Python
itself, and fall back to the backport if Python doesn't provide it.

Since this could mean an additional dependency on some platforms, we
should also declare that dependency.

Signed-off-by: Scott K Logan <[email protected]>

Author: cottsay

launch_ros/launch_ros/actions/node.py

@@ -25,7 +25,10 @@
 from typing import Tuple  # noqa: F401
 from typing import Union
 
-import importlib_metadata
+try:
+    import importlib.metadata as importlib_metadata
+except ModuleNotFoundError:
+    import importlib_metadata
 
 from launch.action import Action
 from launch.actions import ExecuteProcess

launch_ros/package.xml

@@ -16,6 +16,7 @@
   <depend>lifecycle_msgs</depend>
   <depend>osrf_pycommon</depend>
   <depend>rclpy</depend>
+  <depend>python3-importlib-metadata</depend>
   <depend>python3-yaml</depend>
   <depend>composition_interfaces</depend>