ExpandResource must only check the specific ModuleInstances in the
requested path, because the resource may not have been registered yet in
all module instances.
It turns out that within terraform resource expansion will normally
happen with an absolute address. This is because in order to evaluate
the expansion expression, we need to already have the context within a
module instance.
This leaves the existing ExpandResource logic in place as
ExpandModuleResource since it's working, and in case we do find a
location where it's useful to get a full expansion (which may be needed
for instance dependency tracking)
Reword some of the resource related arguments and comments, as they were
copied from the module methods and not entirely accurate.
This package aims to encapsulate the module/resource repetition problem
so that Terraform Core's graph node DynamicExpand implementations can be
simpler.
This is also a building block on the path towards module repetition, by
modelling the recursive expansion of modules and their contents. This will
allow the Terraform Core plan graph to have one node per configuration
construct, each of which will DynamicExpand into as many sub-nodes as
necessary to cover all of the recursive module instantiations.
For the moment this is just dead code, because Terraform Core isn't yet
updated to use it.
