Non-uniform treatment of toplevel modules and submodules

[`Asetmap`](http://docs.mirage.io/odoc/asetmap/Asetmap/index.html) has no toplevel `module Asetmap : sig ... end` definition. 

[`Asetmap.Set`](http://docs.mirage.io/odoc/asetmap/Asetmap/Set/index.html) has a `module Set : sig end` definition. 

This should be unified, I would be in favor of dropping the `module Set : sig end` as it is kind of redundant.
