ament_python does not validate package.xml syntax fully

[RMichaelSwan]

Unlike ament_cmake, ament_python (perhaps more related to not using CMakelists.txt?) will miss issues in the package.xml file which can have confusing downstream results.

For example, you are allowed to have open tags, incorrect closing tags, and likely many other things without triggering any build errors:

 <description>Some package</other_tag>
 <description>Some package with no closing tag

The resulting behavior is that dependencies are skipped (colcon build --packages-up-to <pkg> won't behave) as they are often after description and other similar tags. A package can seem to build and install correctly, but will not be found by ros2 run or ros2 pkg (though it will still be found by colcon cd).

Replicated on ros humble and jazzy (after spinning my wheels trying to figure out why my package wouldn't launch for a while)