prevent pip dependency retrieval, rely on rosdep instead.

[asmodehn]

We tend to not have tests in installed packages anymore, to reduce maintenance costs... debatable but that is the usual de facto practice currently.

So we need to make sure the dependencies in devel workflow (where we test) are the same as the final environment where the package will be installed.

However one of the main point of catkin_pip in a devel workspace, is to retrieve all recent versions from PyPI to make sure everything works together.

Previously, we use to run tests again in the install workspace with ros dependencies to find out any potential problems there. However this is not really doable in practice, which means that in the devel space, we need a way to skip the installation of python dependencies to rely on installed ROS packages OR to pin dependencies version to the same version as the ROS package.

Note that pinning to ROS version is already the preferred approach in Python packages when testing for a specific ROS distro.

So in the actual ROS package release process, we should use an option that uses dependencies from rosdep, and does not retrieve dependencies with pip. This would be somewhat of a special case for an automated ROS build test, as it is useful to get pip dependencies during development.

Such a feature needs to be identified and if needed, implemented in catkin_pip.