Fix forward-slash vs back-slash confusion in Win32
Fix forward-slash vs back-slash confusion in Win32

In windows C:\a\b and C:/a/b refer to the same location, but @INC.unique thinks
they are different, so we canonicalize all paths to C:\a\b on windows.

Fixes the issue in https://gist.github.com/retupmoca/01d09301ac7fc3fd2013

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