Android: Make search for import path case insensitive on Windows

The file system is case insensitive on Windows, and if you e.g.
specified a lower case drive letter in your prefix when building
Qt, then we would bail out because we would think the path
of any given import was located outside the import paths specified.

Task-number: QTBUG-41012
Change-Id: I22a1be32a221cd754dfed93e125d685bade65828
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>