xdist chooses the wrong package version

When installing two different versions of a package in the same environment, xdist doesn't pick the same version as regular pytest.

Steps to reproduce using Invoke package for instance:

virtualenv ENV && source ENV/bin/activate
pip install pytest pytest-xdist invoke==0.11.0

Download the source of Invoke 0.12.2 and install it with python setup.py install.

I have a dummy test file test_version.py, containing:

import invoke

def test_version():
    assert False, str(invoke.__version__)

When running py.test test_version.py, we see that the test is running with Invoke 0.12.2:

============================ test session starts =============================
platform darwin -- Python 2.7.10, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /Users/jcruy/Desktop/xdist_bug, inifile: 
plugins: xdist-1.14
collected 1 items 

test_version.py F

================================== FAILURES ==================================
________________________________ test_version ________________________________

    def test_version():
>       assert False, str(invoke.__version__)
E       AssertionError: 0.12.2
E       assert False

test_version.py:5: AssertionError
========================== 1 failed in 0.09 seconds ==========================

However, when running it with xdist (py.test test_version.py -n 2), we see that the test is running with Invoke 0.11.0:

============================ test session starts =============================
platform darwin -- Python 2.7.10, pytest-2.9.2, py-1.4.31, pluggy-0.3.1
rootdir: /Users/jcruy/Desktop/xdist_bug, inifile: 
plugins: xdist-1.14
gw0 [1] / gw1 [1]
scheduling tests via LoadScheduling
F
================================== FAILURES ==================================
________________________________ test_version ________________________________
[gw1] darwin -- Python 2.7.10 /Users/jcruy/Desktop/xdist_bug/ENV/bin/python
def test_version():
>       assert False, str(invoke.__version__)
E       AssertionError: 0.11.0
E       assert False

test_version.py:5: AssertionError
========================== 1 failed in 0.45 seconds ==========================

Notes:

• I chose the Invoke package for example's sake only, it happens with other packages as well.
• I know that it's not a really good thing to install two versions of a packages in the same environment, but I believe that the behaviour between pytest and xdist should at least be consistent.
• I would gladly contribute to a fix, if you point me to the area of the code which might be responsible for that.

Environment:

• OS X 10.11.4
• Python 2.7.10
• pytest 2.9.2
• pytest-xdist 1.14

Reproduce the mismatch with test_version.py using the reported virtualenv, Invoke versions, pytest command, and -n 2 xdist command. Compare the package version selected by regular pytest and each xdist worker; done means both execution modes consistently select the same version.