[TypeSpec Requirement] Add awareness of "service groups"

https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/directory-structure.md#advanced-scenario-service-group

Example PR adding a sub-service:

https://github.com/Azure/azure-rest-api-specs-pr/pull/17717/checks?check_run_id=24311732426

A possible fix would be, if we don't find `specification/foo/resource-manager/Microsoft.Foo/Bar/stable`, we would fallback and look for `specification/foo/resource-manager/Microsoft.Foo/stable`. If this parent stable folder exists, pass the check.