- handle n-to-1 updates: mostly need to allow for a list of replaced packages.
The tricky part is that the replaced packages will often have 
inter-dependencies, and we must ignore those inter-dependencies.
- UStar::write method, so that we can handle long links, and that we can
write a decent pkg_merge.
- rewrite of PackageLocator to make listing of distant packages effective,
which is the first step of pkg updates.
- finish pulling the pkg_add code into Add.pm/Update.pm.