configs: Emit a warning for backends in nested modules

[apparentlymart]

Terraform considers backend configurations only in the root module, so any declarations in child modules are entirely ignored.

To avoid users mistakenly thinking that a root module backend configuration has taken effect, we'll now emit a warning about it. This is a warning rather than an error because it's reasonable to call a module that would normally be a root module instead as a child module when writing a wrapper module to handle integration testing.

Warning: Backend configuration ignored

  on child/nested-backend-config-child.tf line 2, in terraform:
   2:   backend "ignored" {}

The selected backend applies to the entire configuration, so Terraform expects
provider configurations only in the root module.

This is a warning rather than an error because it's sometimes convenient to
temporarily call a root module as a child module for testing purposes, but
this backend configuration block will have no effect.

This closes #25933, by giving better feedback that ignoring the child module backends is by design.

[codecov]

Codecov Report

Merging #26954 (fb858ec) into master (b06db96) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
configs/config_build.go	81.81% <100.00%> (+2.65%)	⬆️
terraform/evaluate.go	53.30% <0.00%> (+0.41%)	⬆️
dag/marshal.go	54.79% <0.00%> (+1.36%)	⬆️

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