Allow COV_CORE_SOURCE to be empty (it'd be converted to None). Also update docs regarding using pytest-cov with other pytest plugins.

Author: ionelmc

docs/plugins.rst

@@ -14,9 +14,9 @@ Alternatively you can have this in ``tox.ini`` (if you're using `Tox <https://to
 
     [testenv]
     setenv =
-        COV_CORE_SOURCE={toxinidir}/src
+        COV_CORE_SOURCE=
         COV_CORE_CONFIG={toxinidir}/.coveragerc
-        COV_CORE_DATAFILE={toxinidir}/.coverage.eager
+        COV_CORE_DATAFILE={toxinidir}/.coverage
 
 And in ``pytest.ini`` / ``tox.ini`` / ``setup.cfg``::
 

src/pytest_cov/embed.py

@@ -53,7 +53,7 @@ def init():
         import coverage
 
         # Determine all source roots.
-        if cov_source == os.pathsep:
+        if cov_source in os.pathsep:
             cov_source = None
         else:
             cov_source = cov_source.split(os.pathsep)