convert xacro to pure python package to allow running in Windows

[wmmc88]

closes #302

[wmmc88]

Tested locally and running via ros2 run xacro xacro now works on windows

[rhaschke]

Thanks for providing a potential fix for #302. However, I wasn't expecting such a huge changeset. Switching the build type from ament_cmake to ament_python is not an option as we provide some cmake functionality to other packages via xacro-extras.cmake. Can you think of another option? What is the actual culprit of #302, i.e. what's the difference of installed files?

[wmmc88]

Thanks for providing a potential fix for #302. However, I wasn't expecting such a huge changeset. Switching the build type from ament_cmake to ament_python is not an option as we provide some cmake functionality to other packages via xacro-extras.cmake. Can you think of another option? What is the actual culprit of #302, i.e. what's the difference of installed files?

My understanding is that in windows, running a python script like an executable is just not possible like it is in linux. Linux seems to be able to do this because it interprets the shebang and that's not something that windows does. All of the rqt packages(ie. rqt_graph, rqt_console, rqt_plot, rqt_action, etc. ) solved this issue by converting their packages to a pure python package so that colcon would be able to use the setup.py to create and entrypoint script that is invokable via ros2 run. A side effect of this seems to be that none of those packages can be directly invoked anymore... only able to run through ros2 run and that seems to be the intended intention by the ros maintainers of those packages.

It seems like ament_python_install_package was enhanced recently in ament/ament_cmake#328 and offers a much better solution. new approach in #304.