fix: validate implied provider names in submodules

[radeksimko]

Fixes #31572

Example config

(which triggers the bug)

./main.tf

module "test" {
  source = "./sub"
}

./sub/main.tf

resource "-test_test" "name" {
}

Before

After

Notes

There are also cases which may produce an extra error - assuming that the provider name is also invalid for HCL:

I didn't spend much time on deduplicating it though as I'd expect/hope this is generally an error people will see quite rarely.

I chose to quote the original provider name in the error message, although the other existing diagnostic doesn't do that (uses %s instead of %q) - let me know if you prefer either. My preference for %q was primarily driven by making empty name and other short names with just special characters more visible and obvious.

[radeksimko]

I was somewhat confused at first that all the modules in testdata have essentially fake Dir - if each dir was otherwise terraform inited directly, there wouldn't be testdata/invalid-names-in-submodules but only the last part (sub).

I'm not sure if that's intentional or not, just something I wanted to point out here.

[github-actions]

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

[radeksimko]

@jbardin Is it worth back-porting this bug fix? Not sure what your convention is around this and how do you do it.

[jbardin]

d'oh, I forgot to check for a backport label, but yes it's a simple fix causing crashes in the release, so that should be OK. if you could manually backport it to v1.2 that would be great!

In the future, if you apply a backport label before merging, the backport assistant will cherrypick the commits into a new PR for you.

[radeksimko]

@jbardin See #31590

[github-actions]

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