[feature request] "replaces" keyword

[RJVB (René Bertin)]

The Debian/Ubuntu package manager has a feature that allows packages to register as a replacement for another package. This is a more direct complement to the MacPorts replaced_by feature.

I see a few points of interest to a "this port replaces this and that other port(s)":

• it's not an EOL property; for instance, port:foo and port:foo-devel could both use replaces foo-devel and replaces foo respectively. This would streamline the process of switching between a port and its "devel" counterpart because the install/upgrade (or even activation) procedure for the one would first deactivate the other.
• it should also streamline upgrades when a new port replaces more than a single port.

port:xorg-xorgproto is probably a good example for that second point. I had some issues when I did a (partial) upgrade using port upgrade outdated and "xorg-*proto". If xorg-xorgproto had registered the replaces list of all proto ports it replaces, those ports would all have been deactivated at once, when the first to-be-replaced proto port upgrade was being processed.