chore: hardcode unix and threads library in bootstrap

Signed-off-by: Ali Caglayan <alizter@gmail.com>

Author: Alizter

boot/libs.ml

@@ -1,4 +1,4 @@
-let external_libraries = [ "unix"; "threads" ]
+let external_libraries = [ "threads.posix" ]
 
 let local_libraries =
   [ ("otherlibs/ordering", Some "Ordering", false, None)

src/dune_rules/bootstrap_info.ml

@@ -64,11 +64,14 @@ let rule sctx ~requires_link =
         ])
     |> Action_builder.of_memo
   in
-  let externals =
+  (* These are auto-generated *)
+  let _externals =
     List.filter_map externals ~f:(fun lib ->
       let name = Lib.name lib in
       Option.some_if (Lib_name.equal name (Lib_name.of_string "threads.posix")) name)
   in
+  (* These are hardcoded *)
+  let externals = List.map ~f:Lib_name.of_string [ "unix"; "threads" ] in
   Format.asprintf
     "%a@."
     Pp.to_fmt