Our documentation for ModuleCall originally asserted that we didn't need
AbsModuleCall because ModuleInstance captured the same information, but
when we added count and for_each for modules we introduced
ModuleCallInstance to represent a reference to an instance of a local
module call, and now _that_ is the type whose absolute equivalent is
ModuleInstance.
We previously had no absolute representation of the call itself, without
any particular instance. That's what AbsModuleCall now represents,
allowing us to be explicit about when we're talking about the module block
vs. instances it declares, which is the same distinction represented by
AbsResource vs. AbsResourceInstance.
Just like with AbsResource and AbsResourceInstance though, there is
syntactic ambiguity between a no-key call instance and a whole module call,
and so some codepaths might accept both to start and then use other
context to dynamically choose a particular interpretation, in which case
this distinction becomes meaningful in representing the result of that
decision.
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