Can't import module types containing GADTs

Code:
```ml
(* foo.ml *)
module type T = sig
    type _ t = None: unit t
end

(* bar.ml *)
module type T = [%import: (module Foo.T)]
```
Result:
```
File "bar.ml", line 1, characters 34-39:
1 | module type T = [%import: (module Foo.T)]
                                      ^^^^^
Error: The type variable name '_ is not allowed in programs
```