Using `@@` to refer to root module from external `MODULE.bazel` no longer works in 7.1.0

### Description of the bug:

In Bazel 7.0.2 we patch a third-party module and add our own extension. We load this from the root workspace by using:
```
our_extension = use_extension("@@//path/to:our_extenstion.bzl", "our_extension")
```

In Bazel 7.1.0 we now see this error when we attempt this:
```
Error computing the main repository mapping: invalid label for module extension found at @@third_party_module~//:MODULE.bazel:8:22: no repo visible as '@' here
```

I [asked on Bazel Slack](https://bazelbuild.slack.com/archives/C014RARENH0/p1710782864184759) and was told to open a new issue here.

### Which category does this issue belong to?

Core

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

See above. Just using `@@` in an external `MODULE.bazel` will repro.

### Which operating system are you running Bazel on?

Ubuntu 22.04.3 LTS

### What is the output of `bazel info release`?

release 7.0.2 is where it works. release 7.1.0 is where it no longer works

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

N/A

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text
This is in an internal repo.
```

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

I haven't bisected it but it is a regression.

### Have you found anything relevant by searching the web?

No.

### Any other information, logs, or outputs that you want to share?

N/A

Start with the external MODULE.bazel case that calls use_extension("@@//path/to:our_extenstion.bzl", "our_extension") and reproduce it on Bazel 7.0.2 and 7.1.0. Trace the main repository mapping error and compare the handling of @@ root-module labels; done means the external MODULE.bazel can resolve the root extension again without the reported error.

Written by the indexing model from the issue text.