Improve recursive module usage warnings

[lpw25]

Usage warnings are broken for recursive modules where a module or one of its components are only used in the signatures of other modules. For example,

module M : sig end = struct
  module rec Foo : sig
    type t
    val create : Bar.t -> t
  end = struct
    type t = unit

    let create _ = ()
  end

  and Bar : sig
    type t
  end = struct
    type t = unit
  end

  let _ = Foo.create
end;;
[%%expect{|
Line 14, characters 4-10:
14 |     type t
         ^^^^^^
Warning 34: unused type t.
Lines 13-17, characters 2-5:
13 | ..and Bar : sig
14 |     type t
15 |   end = struct
16 |     type t = unit
17 |   end
Warning 60: unused module Bar.
module M : sig end

This PR fixes the problem for the recursive modules themselves. The problem remains for the components of recursive modules. (i.e. the unused module warning above is fixed but the unused type warning is not).

[trefis]

LGTM.

The location change that we can see in the tests seems fine.

[lpw25]

Rebased and changes entry added. Merging when CI is green.