backend/local: treat output changes as side-effects to be applied

[apparentlymart]

This is a baby-step towards an intended future where all Terraform actions which have side-effects in either remote objects or the Terraform state can go through the plan+apply workflow.

This initial change is focused only on allowing plan+apply for changes to root module output values, so that these can be written into a new state snapshot (for consumption by terraform_remote_state elsewhere) without having to go outside of the primary workflow by running terraform refresh. This applies both to direct changes to output blocks and to indirect changes via other objects used in an output's value expression.

This is also better than terraform refresh because it gives an opportunity to review the proposed changes before applying them, as we're accustomed to with resource changes.

The downside here is that Terraform Core was not designed to produce accurate changesets for root module outputs. Although we added a place for it in the plan model in Terraform 0.12, Terraform Core currently (intentionally, to reduce 0.12 release scope) produces inaccurate changesets there which don't properly track the prior values.

We're hoping to rework Terraform Core's evaluation approach in a forthcoming release so it would itself be able to distinguish between the prior state and the planned new state to produce an accurate changeset, but this commit introduces a temporary stop-gap solution of implementing
the logic up in the local backend code, where we can freeze a snapshot of the prior state before we take any other actions and then use that to produce an accurate output changeset to decide whether the plan has externally-visible side-effects and render any changes to output values.

This temporary approach should be replaced by a more appropriately-placed solution in Terraform Core in a later release, which should then allow further behaviors in similar vein, such as user-visible drift detection for resource instances.

This is a partial implementation of #15419, so it will not close that issue but it will achieve one aspect of it and deal with a specific pain-point when making changes to outputs.

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This also includes two minor corrections to the documentation pages for terraform plan and terraform apply which I spotted while I was looking there to see how to note what Terraform considers to be a change.

[codecov]

Codecov Report

Merging #25047 into master will decrease coverage by 0.00%.
The diff coverage is 57.37%.

Impacted Files	Coverage Δ
command/format/diff.go	85.71% <0.00%> (-2.74%)	⬇️
plans/objchange/action.go	0.00% <0.00%> (ø)
backend/local/backend_apply.go	39.33% <66.66%> (+0.40%)	⬆️
backend/local/backend_plan.go	73.02% <85.89%> (+5.19%)	⬆️
command/show.go	55.94% <100.00%> (ø)
terraform/node_resource_plan.go	91.80% <0.00%> (-1.64%)	⬇️
terraform/evaluate.go	53.60% <0.00%> (+0.45%)	⬆️

[ghost]

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