avoid double-instantiation of PluginManager in case of the "python pytest.py" or -m pytest invocation

pytest.py

@@ -1,13 +1,15 @@
 """
-unit and functional testing with Python.
+pytest: unit and functional testing with Python.
 """
 __all__ = ['main']
 
-from _pytest.core import main, UsageError, _preloadplugins
-from _pytest import core as cmdline
-from _pytest import __version__
-
 if __name__ == '__main__': # if run as a script or by 'python -m pytest'
-    raise SystemExit(main())
+    # we trigger the below "else" condition by the following import
+    import pytest
+    raise SystemExit(pytest.main())
 else:
+    # we are simply imported
+    from _pytest.core import main, UsageError, _preloadplugins
+    from _pytest import core as cmdline
+    from _pytest import __version__
     _preloadplugins() # to populate pytest.* namespace so help(pytest) works