Although addrs.Target can in principle capture the information we need to
represent move endpoints, it's semantically confusing because
addrs.Targetable uses addrs.Abs... types which are typically for absolute
addresses, but we were using them for relative addresses here.
We now have specialized address types for representing moves and probably
other things which have similar requirements later on. These types
largely communicate the same information in the end, but aim to do so in
a way that's explicit about which addresses are relative and which are
absolute, to make it less likely that we'd inadvertently misuse these
addresses.
This PR adds decoding for the upcoming "moved" blocks in configuration. This code is gated behind an experiment called EverythingIsAPlan, but the experiment is not registered as an active experiment, so it will never run (there is a test in place which will fail if the experiment is ever registered).
This also adds a new function to the Targetable interface, AddrType, to simplifying comparing two addrs.Targetable.
There is some validation missing still: this does not (yet) descend into resources to see if the actual resource types are the same (I've put this off in part because we will eventually need the provider schema to verify aliased resources, so I suspect this validation will have to happen later on).
