- 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.
- clean up the interface of the dependencies module, to be able to use it in
n-to-1 updates.
- in interactive mode, make it ask questions whenever the user could get
frustrated.
- finish pulling the pkg_add code into Add.pm/Update.pm.
- figure out when to continue after some errors, which should not stop a
full update, for instance... or ignore already installed packages.
- make sure all user-interface is done through a few functions, so that it's
easy to plug another interface in.
- add needed annotations to handle updates correctly.