Stdlib__ name leaking

ocp-index-top leaks module names with two underscores that should be hidden from users, for example:

```
utop [1]: #doc List.map
Stdlib__list.map val ('a -> 'b) -> 'a list -> 'b list
    [List.map f [a1; ...; an]] applies function [f] to [a1, ..., an],
    and builds the list [[f a1; ...; f an]]
    with the results returned by [f].  Not tail-recursive.
```