Fix linter errors from #131.

[nuclearsandwich]

This fixes the linters which have been failing on nightlies over the weekend since #131.

It may be that we don't care about these issues or have different preferences for resolving them in which case this PR may be amended to provide a different solution.

[wjwwood]

Thanks!

[dirk-thomas]

Can either of you reproduce the behavior of the buildfarm regarding the import order? Locally the previous state passed the linter and the new state doesn't. And from the changelog it looks like lexicographic ordering is expected (favoring the previous state).

[dirk-thomas]

Nvm, after an update the config wasn't working anymore. It needs: ignore = I202 and import-order-style = google to reproduce the behavior in my editor (basically our custom configuration).

[nuclearsandwich]

Yeah the sort ordering was what I was most suspicious about. I was able to reproduce both errors locally running the tests via colcon test. I usually end up copying or linking the config from ament_flake8 into ros2 projects I'm working on so that editor based feedback matches the buildfarm linter configuration.