Return right slashes for package dirname on Windows.
Return right slashes for package dirname on Windows.

FindNumpy works inconsistently in Windows and we suspect it might be caused by the non-Windowsy slashes being returned by dirname.
UCL/GreatCMakeCookOff,UCL/GreatCMakeCookOff,UCL/GreatCMakeCookOff,UCL/GreatCMakeCookOff