Can't find module alias defined in module

RBS unexpectedly does not resolve the following module name.


```rbs
module M
  module N
  end

  module N2 = N
end

module M::N3 = M::N

class C
  include M::N     # ok
  include M::N2    # It causes `Could not find mixin: M::N2 (RBS::NoMixinFoundError)`
  include M::N3    # ok
end
```

I expect RBS to resolve all module names, but it raises an error on `M::N2`.


