Backport of init: detect and report invalid submodules into v1.9

[github-actions]

Backport

This PR is auto-generated from #35839 to be assessed for backporting due to the inclusion of the label 1.9-backport.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@liamcervante
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "2a2410d1bcc04c5e977308e0826bbb39f3739534", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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This PR updates terraform init so that if a module source is requesting a subdirectory within a registry module, where the registry module itself does exist and is successfully downloaded but the subdirectory is missing, Terraform correctly highlights the missing subdirectory as the problem rather than a generic "This is a bug in Terraform" message.

Before

╷
│ Error: Unreadable module directory
│ 
│ The directory .terraform/modules/karpenter_fg_profile/modules/fargate_profile could not be read. This is a bug in Terraform and should be reported.
╵

After

╷
│ Error: Unreadable module subdirectory
│ 
│ The directory .terraform/modules/karpenter_fg_profile/modules/fargate_profile does not exist. The target submodule modules/fargate_profile does not exist within the target module.
╵

Fixes #35836

Target Release

1.9.x

Draft CHANGELOG entry

BUG FIXES

• terraform init: Highlight missing subdirectories of registry modules in error message.

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Overview of commits

• 2f6fe3e

• 2a2410d

[liamcervante]

This cherry-pick is actually fine - it only failed the merge that was needed to make unrelated tests pass. These tests were never merged to this branch.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
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