References to outside of module types must be fully qualified

[Julow]

module X : sig

  module type T = sig
    (** {!t} *)
  end

  type t

end

module Y : X.T

The reference that's inside X.T won't resolve in the expansion of Y.

Annoyingly, the error message points to the reference and not to Y at all:

File "test.mli", line 4, characters 8-12:
Failed to resolve reference unresolvedroot(t) Couldn't find "t"

[Julow]

A similar issue is synopsis that contain references, these references won't resolve when shown in the parent page. These references must be fully-qualified and might not be resolved: https://github.com/ocaml/odoc/pull/658

Perhaps the same rule can be applied to references in module types that point to outside of the module type ? (suggested by @jonludlam )

[jonludlam]

Module types are where these issues are likely to show up the most, but the same problem applies anywhere where we build expansions. The problem is that the references are all resolved after the expansion has happened, so the original context is gone. Stdlib currently has a few of these sorts of issues - e.g. https://github.com/ocaml/ocaml/blob/45612f14afc1abaa19d8585ec1f48c3cbbfbc652/stdlib/map.mli#L242

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