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.
Edited by Tobias Kloss