SystemTools: Fix FileIsSymlink with Windows data deduplication

On Windows machines with enabled data deduplication files duplicates are
incorrectly recognized as symlinks and therefore install fails with
"file COPY cannot read symlink".

Similar issues can be found here:

* https://github.com/Microsoft/vcpkg/issues/2488
* https://github.com/puppetlabs/puppet/pull/6513

From the later fix this commit is mainly influenced.