dune_python_install_package can only install packages without setup.py

Before the python overhaul, the dune_python_install_package was intended to install complete python packages other than dune. This is not possible anymore. The reasons and possible solutions are:

1. Python projects are expected to have a setup.py.in file or a generic one will be provided. This does not hold for well-formed python projects because they have other own setup.py and the CMake generated setup.py is not appropriate. f954a5a1 is offending commit where the path to the python project is unconditionally set to the binary directory instead of the source directory. A solution is to set the PYINST_FULLPATH to the binary directory only if no setup.py is provided.
2. The pip install is called with a --no-index argument. While this is fine for dune projects, this is not ideal for generic project installations where the setup.py states dependencies expected to be fulfilled by pip. A solution for this would be to have an opt-in option to have index (e.g. USE_INDEX).