Install generated Python interfaces in a Python package

[hidmic]

This makes Python interfaces discoverable through pkg_resources and importlib.metadata.

Full CI to be safe:

• Linux
• Linux-aarch64
• macOS
• Windows

[jacobperron]

This change prevents a package from building if it both generates interfaces and installs a Python package. For example, I have a package that is not longer building since it does something like this:

rosidl_generate_interfaces(${PROJECT_NAME}_msgs
  ...
)

ament_python_install_package(${PROJECT_NAME})

I see an error like:

CMake Error at /home/jacob/ros2-linux/share/ament_cmake_python/cmake/ament_python_install_package.cmake:108 (add_custom_target):
  add_custom_target cannot create target
  "ament_cmake_python_symlink_my_package" because another target with the
  same name already exists.  The existing target is a custom target created
  in source directory
  ...

[hidmic]

I can't say I didn't see this one coming.

A user-defined ament_python_install_package(${PROJECT_NAME}) contends with rosidl_generate_interfaces() to install the ${PROJECT_NAME} Python package. That was not a problem before because there was no Python package (with metadata, discoverable). Python sources were just incrementally pushed to the install space.

Maybe there is a way for an ament_python_install_package(${PROJECT_NAME}) call to extend or augment previous ones, somehow. We would not be able to tell if that was on purpose or not though.
WDYT @ros2/team ?

[jacobperron]

Maybe there is a way for an ament_python_install_package(${PROJECT_NAME}) call to extend or augment previous ones, somehow. We would not be able to tell if that was on purpose or not though.

If it's possible, we could make it an option to extend a previous call to ament_python_install_package. This way we would know if someone is doing this on purpose. Then the error message could also be improved to suggest using the "extend" option if it detects multiple calls to ament_python_install_package with the same target name.

[jtbandes]

I'm running into this error in rosbridge_suite as well, when trying to build for Rolling: RobotWebTools/rosbridge_suite#581

Is there any recommendation for how to fix or work around this issue?

[jtbandes]

Filed a new issue to track rather than discussing on an old merged PR: #141