Staticcheck fixes in terraform package

[pselle]

Fixes within the terraform package to remove staticcheck errors from running staticcheck github.com/hashicorp/terraform/terraform.

The following notices were fixed:

• Makes error last return
• Unused value
• Unnecessary type transformation
• Errors are lowercase and don't end in punctuation

On the last item, it means that some error appearances will change:

        Plugin reinitialization required. Please run "terraform init".
        
        [... cut for brevity...]
        
        2 problems:
        
        - failed to instantiate provider "registry.terraform.io/hashicorp/unknown" to obtain schema: unknown provider "registry.terraform.io/hashicorp/unknown"

(These bulleted items are now lowercase)

[codecov]

Codecov Report

Merging #27429 (fb5f7b9) into master (0a31fa0) will increase coverage by 0.00%.
The diff coverage is 54.83%.

Impacted Files	Coverage Δ
terraform/transform_import_state.go	87.12% <0.00%> (ø)
terraform/transform_provider.go	81.43% <ø> (-0.08%)	⬇️
terraform/transform_vertex.go	45.45% <0.00%> (ø)
terraform/schemas.go	65.13% <25.00%> (ø)
terraform/node_resource_abstract_instance.go	71.98% <55.55%> (ø)
terraform/node_resource_apply_instance.go	49.76% <100.00%> (ø)
terraform/node_resource_destroy.go	61.79% <100.00%> (ø)
terraform/node_resource_destroy_deposed.go	56.75% <100.00%> (ø)
terraform/node_resource_plan.go	96.11% <0.00%> (-1.95%)	⬇️
internal/providercache/dir.go	73.46% <0.00%> (+6.12%)	⬆️

[jbardin]

Thanks @pselle! The staticcheck CI PR adds a script and make target which skips all the of the "style" related checks, and legacy packages as a starting point.

The error messages originally broke the recommended style, because they were often meant to be displayed as the full message in the UI. Since we now have other means of formatting errors (primarily using tfdiags) we can slowly convert them as needed, but there are a lot to cover.

[pselle]

@jbardin Thanks for the reminder about the staticcheck PR! I'm looking forward to when that lands.

[ghost]

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