core: Specialized errors for incorrect indexes in resource reference

[apparentlymart]

In prior versions of Terraform we permitted inconsistent use of indexes in resource references, but in as of 0.12 the index usage must correlate properly with whether count is set on the resource.

Since users are likely to have existing configurations with incorrect usage, here we introduce some specialized error messages for situations where we can detect such issues statically. This seems to cover all of the common patterns we've seen in practice.

Some usage patterns will fall back on a less-helpful dynamic error here, but no configurations coming from 0.11 can end up that way because 0.11 did not permit forms such as aws_instance.no_count[count.index].bar that this validation would not be able to "see".

Our configuration upgrade tool also contains a fix for this already, but it takes a more conservative approach of adding the index [0] rather than [count.index] because it can't be sure (without human help) if correlation of indices is what was intended.

This addresses #19216 in two ways:

• It gives a more helpful error message for what is in practice a usage error rather than an evaluation error.
• By implementing it as a static check, it will be returned even on the initial terraform apply in that example, whereas in alpha1 we'd catch it only after the resources were initially created and so we could see the incorrect "each mode" in state.

[ghost]

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