Add setting to disable adding cwd to pythonpath

Hello!

I'm trying to use pytest-xdist together with tox to test a python extension written with cython.

When I run tox, it creates a new virtualenv, compiles my project and installs it to ./.tox/.../site-packages/modulename so that there is a directory with source code (./project/modulename) and a directory with compiled python extensions (./project/.tox/.../site-packages/modulename):

./project
  |-setup.py
  |-modulename
  |  |-__init__.py
  |  |-extension.pyx
  |-...
  |-.tox
     |-python/.../site-packages
        |-modulename
           |-__init__.py
           |-extension.so

When running normally, there is ./project/.tox/python/.../site-packages in PYTHONPATH and there is no ./project in it so modulename from site-packages imported when testing.

However, xdist adds ./project to PYTHONPATH (before .tox/.../site-packages) which causes modulename from project root being imported instead of modulename from site-packages. And everything fails with ImportError because there are no .so files in that modulename from project root.

As I understand, the code that adds cwd to pythonpath is here and it seems like there is no way to control this.

I propose adding a setting (a cli parameter) to disable this behavior.

Start with xdist/remote.py around line 149, then trace how pytest-xdist registers and handles command-line options. Add a setting that controls whether the current working directory is added to PYTHONPATH, and verify with a tox plus pytest-xdist setup that disabling it imports the installed extension instead of the source directory.