- smart handling of .libs-*: pkg_add correctly discards potential conflicts.
It doesn't handle yet actual conflicts with .libs-*.
- `global' replacement through pkg_add -r: the dependency solver needs
to look into the command-line to figure out ways to solve dependencies:
in effect, packages updated through -r must be sorted in dependency order.
This does break pkg_add -n -r very thoroughly for complicated cases.
And this is needed for 3.7 updates, if we don't want to have to update
each package manually, in order.
- 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.