Pure-python packages need rebuilt even with symlink-install

[ana-GT]

When you build a package with symlink-install, the expectation is that if you make modifications to the original package, you can run the files modified and the changes should be usable without the need to rebuild. For pure-python packages that use ament-python (setup.cfg, setup.py), this only works for the python scripts. If you modify a launch file, you have to rebuild.

Fix: Make the package use CMake. This is a perfectly okay option that is mostly used for packages that have a combination of C++ and python code, so it is applicable to pure-python pkgs as well, with the added bonus that if we need at some point to add C++ code, we are set to go.

We have a lot of pure-python-packages. A few of them are used in the tutorials, and when I write instructions for users, it is a pain to have to tell people to rebuild stuff everytime an instruction say for instance, edit this launch file. I'd suggest that we convert all python packages to use CMake, but as we are ending the project soon, I'll only convert the packages that are used by the users in the tutorials, for easiness.