Import doesn't work for submodules

[gerdstolpmann]

e.g.

# module M = struct type t = int end;;
module M : sig type t = int end
# type t = [%import: M.t] [@@deriving show];;
Error: [%import]: cannot locate module M

I just ran into the issue because I couldn't add the [@@deriving] clause directly to the module M for formal reasons (a bad interaction with include module type of), so I thought I could derive in another module defined in the same file. Fortunately, I could work around my moving M to a file of its own. Neverthless, this restriction was a surprise.

[whitequark]

Related to #14

[xguerin]

The import logic for submodules is incomplete. Related to #22. I'll look into it.

[Pitometsu]

v. 1.6.2 still the same. Didn't it be fixed in some commit yet?