Fix a bug in Findlib.available

Fix #24

bin/main.ml

@@ -400,7 +400,7 @@ let external_lib_deps =
                in
                let missing =
                  String_map.filter externals ~f:(fun name _ ->
-                   not (Findlib.available context.findlib name))
+                   not (Findlib.available context.findlib name ~required_by:[]))
                in
                if String_map.is_empty missing then
                  acc

src/findlib.ml

@@ -424,9 +424,9 @@ let find t ~required_by name =
   | exception (Package_not_found _) -> None
   | x -> Some x
 
-let available t name =
-  match find_exn t name with
-  | _ -> true
+let available t ~required_by name =
+  match find_exn t name ~required_by with
+  | (_ : package) -> true
   | exception (Package_not_found _) -> false
 
 let closure pkgs =

src/findlib.mli

@@ -34,7 +34,7 @@ type package =
 val find     : t -> required_by:string list -> string -> package option
 val find_exn : t -> required_by:string list -> string -> package
 
-val available : t -> string -> bool
+val available : t -> required_by:string list -> string -> bool
 
 val root_package_name : string -> string
 

src/lib_db.ml

@@ -70,7 +70,7 @@ let top_sort_internals t ~internal_libraries =
 let lib_is_installable t ~from name =
   match find_internal t ~from name with
   | Some (_, lib) -> String_map.mem lib.name t.instalable_internal_libs
-  | None -> Findlib.available t.findlib name
+  | None -> Findlib.available t.findlib name ~required_by:[Utils.jbuild_name_in ~dir:from]
 
 let choice_is_possible t ~from { Lib_dep. lits; _ } =
   List.for_all lits ~f:(function