(PUP-3191) Use FileSystem.readlink in unpacker

Prior to this commit we were using File.readlink, which does not work properly
on Windows platforms, to tell the user about symlinks found when unpacking.
This commit changes that behavior to instead use the Puppet::Filesystem.readlink
function, which should work on all platforms that support symlinks.
Merge pull request #3072 from highb/fix/stable/pup-3191_fix_readlink

(PUP-3191) Use FileSystem.readlink in unpacker