feat(dune_lang): add Ordered_set_lang.Unexpanded.has_include

Mirrors the existing `has_standard`. Callers that need to summarise
an `Unexpanded.t` syntactically — without reading the file an
`(:include FN)` directive points to — must now check `has_include`
and widen their answer accordingly, since `fold_strings` skips
`Include` AST nodes entirely.

Used by the menhir auto-injection in opam-create (ocaml#14434, fixes
ocaml#14428): a `(modules (:include FN))` library would otherwise be
mis-summarised as compiling no modules.

Signed-off-by: Robin Bate Boerop <me@robinbb.com>

Author: robinbb

src/dune_lang/ordered_set_lang.ml

@@ -356,6 +356,17 @@ module Unexpanded = struct
     loop t.ast
   ;;
 
+  let has_include t =
+    let rec loop ast =
+      match ast with
+      | Ast.Include _ -> true
+      | Ast.Standard | Ast.Element _ -> false
+      | Ast.Union l -> List.exists ~f:loop l
+      | Ast.Diff (l, r) -> loop l || loop r
+    in
+    loop t.ast
+  ;;
+
   type position =
     | Pos
     | Neg

src/dune_lang/ordered_set_lang.mli

@@ -72,6 +72,11 @@ module Unexpanded : sig
 
   val has_standard : t -> bool
 
+  (** Whether [t] contains any [(:include FN)] directive. {!fold_strings}
+      skips these, so callers needing a static summary must widen
+      accordingly. *)
+  val has_include : t -> bool
+
   type position =
     | Pos
     | Neg