Symlinks handling

See [code from reprozip](https://github.com/fchirigati/reprozip/blob/master/reprozip/pack/system_tap/parse_stap_out.py#L581)

I correctly create chains of symlinks if a link is referenced, so that its target(s) are created as well in the pack. However, links might be walked over elsewhere in the path to the file; these links need to exist in the pack as well.