install_rules: call dir_contents only when necessary

This prevents path_expander to expand the artifacts too early

Signed-off-by: Marcello Seri <marcello.seri@gmail.com>

src/install_rules.ml

@@ -311,10 +311,10 @@ module Gen(P : Params) = struct
     let entries_per_package =
       List.concat_map (SC.stanzas_to_consider_for_install sctx)
         ~f:(fun { SC.Installable. dir; stanza; kind = dir_kind; scope } ->
-          let dir_contents = Dir_contents.get sctx ~dir in
           match stanza with
           | Library ({ public = Some { package; sub_dir; name = _}
                      ; _ } as lib) ->
+            let dir_contents = Dir_contents.get sctx ~dir in
             List.map (lib_install_files ~dir ~sub_dir lib ~scope
                         ~dir_kind ~dir_contents)
               ~f:(fun x -> package.name, x)
@@ -328,6 +328,7 @@ module Gen(P : Params) = struct
             in
             List.map ~f files
           | Documentation ({ package; _ } as d) ->
+            let dir_contents = Dir_contents.get sctx ~dir in
             List.map ~f:(fun mld ->
               (package.name,
                (Install.Entry.make