Underscore-prefixed models referenced in public APIs are not exported

The modular emitter filters out underscore-prefixed models from exports (see [buildRootIndex.ts](https://github.com/Azure/autorest.typescript/blob/b2525be465f78689eef4d46fc71afa51e4b14ef7/packages/typespec-ts/src/modular/buildRootIndex.ts#L304)) although they are referenced in the public APIs.

For example:
```
model _123Test { // not exported
id: string;
name: string;
}

op createTestData(
testData: _123Test
): _123Test;
```