Previously we fetched schemas during the AttachSchemaTransformer,
potentially multiple times as that was re-run for each graph built. Now
we fetch the schemas just once during context construction, passing that
result into each of the graph builders.
This only addresses the schema accesses during graph construction. We're
still separately loading schemas during the main walk for evaluation
purposes. This will be addressed in a later commit.
Since ProviderTransformer now needs the schema in order to infer indirect
references to providers, we must run AttachSchemaTransformer before the
provider transformers in order to calculate the correct ordering of
operations.
Some of the objects that are referencable from expressions are transient
values computed only during a graph walk, and not persisted in state. In
order to support arbitrary evaluation of expressions, such as in the
"terraform console" CLI command, it's necessary to be able to evaluate
these values before we start evaluating.
This new Eval method achieves this by performing a special graph walk that
ignores resources (except for dependency resolution) and just focuses on
evaluating all of these transient values, before returning an evaluation
scope that can then resolve expressions in terms of that result.
This replaces the Context.Interpolator method, which was fraught with
various issues due to it not properly priming the state before evaluating.
