(PUP-2912) Puppet FS symlinks in build/unpack

Prior to this commit build/unpack were trying to use the Ruby stdlib symlink to
deal with symlinks. This fails on platforms (Windows) that Puppet says supports
symlinks because it does not duck punch Ruby stdlib.

This commit switches the specs and code for build/unpack to use Puppet FS lib's
symlink functions. This should allow it to function correctly on Windows hosts that Puppet
supports symlinks.