[doctests] Namespace packages (PEP 420) missing, leading to errors with relative imports #3396

When testing packages with implicit namespace and using --doctest-module the doc test module is imported without the implicit namespace.

This causes problems as the module can gets imported with different name. Once with the proper path and once with missing the namespace.

My pytest related versions:
pytest (3.5.0)
pytest-attrib (0.1.3)
pytest-cov (2.5.1)
pytest-forked (0.2)
pytest-timeout (1.2.1)
pytest-xdist (1.22.2)

OS is Windows 7.
Please look at the example:

foo.zip