Fix DCE false positives for functors constrained by a named module type

[JonoPrest]

When a functor result is constrained to a module type defined in another file, every value in the functor body is reported dead even when instances are used from live code.

(* gen.ml *)
module type S = sig
  val find_opt : int -> int option
  val unused_in_sig : int -> int
end

(* hash.mli *)
module Make (K : sig type t end) : Gen.S

(* hash.ml *)
module Make (K : sig type t end) = struct
  let find_opt k = Some k                                                                                                                                                     let unused_in_sig k = k
  let truly_dead k = k                                                                                                                                                      end
                                                                                                                                                                            (* main.ml *)
module H = Hash.Make (struct type t = int end)                                                                                                                              let () = ignore (H.find_opt 1)

Before: find_opt and hash.Make are reported dead. Expected: only unused_in_sig and truly_dead.
On OCaml 5.3+ the find_opt binding depends on the uid of val find_opt in gen.ml, but the dependency extractor only loaded the local .cmt/.cmti, so the link was dropped.
Fix:

• Index all cmt files by compilation unit and resolve foreign uids from them. Same-named units (e.g. unwrapped libraries in different directories) are disambiguated by the importer's interface digest; if that fails, every candidate is consulted opped.
• Process value dependencies after all files are scanned. When the signature side is a val in a module type rather than a declaration, forward its references onto the
  implementation, so unused signature items are still reported.

Fixtures cover .mli and inline constraints, plus a duplicate-unit regre

[JonoPrest]

@codex review

[JonoPrest]

I can't stack PRs from my fork owned branches so closed for #217