support setting cmake_source_dir to a relative parent directory

[jcfr]

[jcfr]

@massich Here is the commit message that could be associated with the removal of this code:

setuptools_wrap: Fix handling of relative parent cmake source dir

This commit fixes the case where `cmake_source_dir` is set to a relative directory
outside of the `setup.py` directory. It does so by removing code originally introduced
in 0d0ddbf (setuptools_wrap: Add support for "cmake" and "hybrid" python package)
that was never covered by tests.

Fixes #335