Fix resolving optional public library

This is a fix for the issue ocurrent#53. The public optional libraries were
resolved.

deps.ml

@@ -103,11 +103,11 @@ let resolve_internal_deps d_items items_pkg =
       else begin
         Hashtbl.add acc (get_name item) item;
         (get_item item).internal_deps
-        |> List.filter_map (fun (name, k) ->
+        |> List.filter_map (fun (name, _) ->
             match Hashtbl.find_opt d_items_lib (String.cat name ".lib") with
             | None -> None
             | Some d_item_lib ->
-              if Kind.is_required k && Option.is_some (get_item d_item_lib).package then None
+              if Option.is_some (get_item d_item_lib).package then None
               else Some d_item_lib)
         |> fun internals -> add_internal acc (tl @ internals)
       end

tests/test_optional_public_lib.t

@@ -0,0 +1,66 @@
+Inspired from test_vendoring.t test. Create a project with an public optional libraries
+It fixes the bug #53. The fix is about to not resolve an public optional library when used as dependency.
+
+  $ mkdir bin lib optional
+
+  $ cat > dune-project << EOF
+  > (lang dune 2.7)
+  > (generate_opam_files true)
+  > (package
+  >  (name main)
+  >  (synopsis "Main package")
+  >  (depends libfoo))
+  > (package
+  >  (name optional)
+  >  (synopsis "Optional package")
+  >  (depends bos))
+  > EOF
+
+  $ cat > bin/dune << EOF
+  > (executable
+  >  (name main)
+  >  (package main)
+  >  (public_name main)
+  >  (libraries lib))
+  > EOF
+
+  $ cat > lib/dune << EOF
+  > (library
+  >  (name lib)
+  >  (public_name main)
+  >  (libraries findlib 
+  >   (select file.ml from
+  >    (optional -> file.enabled.ml)
+  >    (         -> file.disabled.ml))))
+  > EOF
+
+  $ cat > optional/dune << EOF
+  > (library
+  >  (name optinal)
+  >  (public_name optional)
+  >  (libraries bos))
+  > EOF
+
+  $ touch bin/main.ml lib/lib.ml lib/file.disabled.ml lib/file.enabled.ml optional/optional.ml
+  $ dune build
+
+Replace all version numbers with "1.0" to get predictable outut.
+
+  $ export OPAM_DUNE_LINT_TESTS=y
+
+Check configuration:
+
+  $ dune external-lib-deps -p main @install
+  dune: This subcommand has been moved to dune describe external-lib-deps.
+  [1]
+
+Check that the missing findlib for "lib" is detected, but not "optional"'s dependency
+on "bos":
+
+  $ opam-dune-lint </dev/null
+  main.opam: changes needed:
+    "ocamlfind" {>= "1.0"}                   [from lib]
+  optional.opam: OK
+  Note: version numbers are just suggestions based on the currently installed version.
+  Run with -f to apply changes in non-interactive mode.
+  [1]