Display a message on Windows if pip is being modified.
Display a message on Windows if pip is being modified.

If the user run a pip modifying command without using the python -m,
pip now displays an error requesting the user to use python -m method of
invoking pip.

xavfernandez/pip,zvezdan/pip,xavfernandez/pip,rouge8/pip,pypa/pip,pfmoore/pip,rouge8/pip,pradyunsg/pip,techtonik/pip,pfmoore/pip,techtonik/pip,RonnyPfannschmidt/pip,zvezdan/pip,techtonik/pip,RonnyPfannschmidt/pip,RonnyPfannschmidt/pip,sbidoul/pip,rouge8/pip,sbidoul/pip,xavfernandez/pip,pypa/pip,pradyunsg/pip,zvezdan/pip