Uncaught exception Odoc_xref2.Tools.UnexpandedTypeOf

Trying to compile the following mli file
```ocaml
(* a.mli *)
module M: sig include module type of String end
```
with
```bash
ocamc -c -bin-annot a.mli
odoc compile --package test a.cmti
```
yields the following error:
> odoc: internal error, uncaught exception:
      Odoc_xref2.Tools.UnexpandedTypeOf

(when there is no `.odoc` file for `String` in the include path).