Fix arg parsing when computing dependencies in roslaunch-check

parse_launch in depends.py isn't building up a context before evaluating args, so any `include`, `node` or `test` tags that reference an arg will fail.

This causes roslaunch-check to crash when parsing complex launch files such as https://github.com/ros-interactive-manipulation/pr2_object_manipulation/blob/groovy-devel/manipulation/pr2_object_manipulation_launch/launch/laser%2Bstereo-perception.launch