Fix coverage paths

Codecov couldn't find the coverage files, perhaps because this is a more complex workspace than usual, and thus really needs to be given the path to the files generated by lcov and coveragepy. I switched to the pip package since it's a bit nicer to use compared to the bash script.

Some coverage is missing for some installed headers in tracetools due to an issue/limitation with colcon-lcov-result, see https://github.com/colcon/colcon-lcov-result/issues/9. We could run lcov manually (e.g. https://github.com/ros-planning/navigation2/pull/553/files), but for now we'll keep this.

I've also filtered out the tracetools_test/test/ directory since we don't want coverage information of test code.

Overall it's not perfect, but we can make it better whenever we work on boosting coverage.

Finally, I added a codecov badge to the README to display the coverage %.