prepend editable package path to python path

[asmodehn]

Looking at pyros-dev/pyros-setup#46 it seems we do need to have catkin_pip and pyros-setup totally independent, and not relying on anything from the other in any case.

So we would have to reactivate and fix that code : https://github.com/asmodehn/catkin_pip/blob/devel/cmake/env-hooks/42.site_packages.sh.develspace.in#L52

[asmodehn]

currently in progress in https://github.com/asmodehn/catkin_pip/tree/gopher-devel

[asmodehn]

This works the first time (PYTHONPATH doesnt have any path of editable packages.
But the following times we hit : pypa/setuptools#1005

[asmodehn]

currently on hold in there : https://github.com/asmodehn/catkin_pip/tree/fix_editable

[asmodehn]

Note : we need to rely on this features to workaround problems with namespace packages install. watch https://github.com/jonparrott/namespace-pkg-tests/blob/master/table.md (from pypa/pip#3)

[asmodehn]

We also need this to be able to get source version of pip packages (linked with eggs) BEFORE we get the installed ROS/system version. Python by default will still put PYTHONPATH content before egg-links/editable packages path in sys.path.