Stop requiring multi-vars (splats) to be in array brackets

[apparentlymart]

Prior to Terraform 0.7, lists in Terraform were just a shallow abstraction on top of strings with a magic delimiter between items. Wrapping a single string in brackets in the configuration was Terraform's prompt that it needed to split the string on that delimiter during interpolation.

In 0.7, when first-class lists were added, this convention was preserved by flattening lists-of-lists by one level when they were encountered in configuration. However, there was an oversight in that change where it did not correctly handle the case where the inner list was unknown.

In #14135 we removed some code that was flattening partially-unknown lists into fully-unknown (untyped) values. This inadvertently exposed the missed case from the previous paragraph, causing issues for list-wrapped splat expressions with unknown members. While this worked fine for resources, due to some fixup done inside helper/schema, this did not work for other interpolation contexts such as module blocks.

Various attempts to fix this up and restore the flattening behavior selectively were unsuccessful, due to a proliferation of assumptions all over the core code that would be too risky to change just to fix this bug.

This change, then, takes the different approach of removing the requirement that splats be presented inside list brackets. This requirement didn't make much sense anymore anyway, since no other
list-returning expression had this constraint and so the rest of Terraform was already successfully dealing with both cases.

This leaves us with two different scenarios:

• For resource arguments, existing normalization code in helper/schema does its own flattening that preserves compatibility with the common practice of using bracketed splats. This change proves this with a test within the "test" provider that exercises the whole Terraform core and helper/schema stack that assigns bracketed splats to list and set attributes.

• For arguments in other blocks, such as in module callsites, the interpolator's own flattening behavior applies to known lists, preserving compatibility with configurations from before partially-computed splats were possible, but those wishing to use partially-computed splats are required to drop the surrounding brackets. This is less concerning because this scenario was introduced only in 0.9.5, and was failing in this scenario anyway.

As of this commit, the recommendation is to stop using brackets around splats but the old form continues to be supported for backward compatibility. In a future major version of Terraform we will probably
phase out this legacy form to improve consistency, but for now both forms are acceptable at the expense of some (pre-existing) weird behavior when actual lists-of-lists are used.

This addresses #14521 by officially adopting the suggested workaround of dropping the brackets around the splat. However, it doesn't yet allow passing of a partially-unknown list between modules: that still violates assumptions in Terraform's core, so for the moment partially-unknown lists work only within a single interpolation expression, and cannot be passed around between expressions. Until more holistic work is done to improve Terraform's type handling, passing a partially-unknown splat through to a module will result in a fully-unknown list emerging on the other side, just as was the case before #14135; this change just addresses the fact that this was failing with an error in 0.9.5.

[ghost]

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