Only create the first instance of a file shared between packages
- When packages share an identical file, we only need to create the
first one we encounter and skip it for the rest (reverse already
happens, and must happen, on erase). This has the benefit of
getting the timestamp to match the first file, which often is
also the last one to get removed. Also when lots of shared files
are involved (such as in multilib installations), this can give
several percents worth of speed gain as we dont unnecessarily rewrite
the same contents over and over.