cc runfiles - `BAZEL_CURRENT_REPOSITORY` should be `_main` when run from the root module

### Description of the feature request:

When you use runfiles as per the documentation in [runfiles.h](https://github.com/bazelbuild/rules_cc/blob/0.1.1/cc/runfiles/runfiles.h), `BAZEL_CURRENT_REPOSITORY` turns out to be:

- Module name of the 3P library that has the code if the usage is in a 3P library - good 👍🏾 working as expected.
- empty if run from the root module. - ❌ not intuitive.

**Request:**
Return `BAZEL_CURRENT_REPOSITORY` as `_main` instead of returning empty when run from the root module. That also easily resolves runfiles from that path.

### Which category does this issue belong to?

Core

### What underlying problem are you trying to solve with this feature?

**Problem:**
The runfile object doesn't exist at the path that I construct from `BAZEL_CURRENT_REPOSITORY` as the files are present in `_main` instead of `` (empty path) for the root module.

**Workaround:**
The current workaround is to catch the empty, interpret it as `_main` but this is undocumented in the header file and/or confusing.

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

linux x86_64

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

release 8.1.1

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

N/A - bazelisk

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

```text
N/A - private repo
```

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

_No response_

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

I'm creating the bug here instead of `rules_cc` because as I understand, the value of `BAZEL_CURRENT_REPOSITORY` is provided by bazel itself (and _not_ rules_cc)

Cc: @meteorcloudy

Start with the runfiles documentation in cc/runfiles/runfiles.h and trace how Bazel provides BAZEL_CURRENT_REPOSITORY for code in the root module versus a third-party module. Identify the relevant Bazel entry point and tests, then verify that root-module lookups use _main and that the existing third-party behavior remains unchanged.

Written by the indexing model from the issue text.