Don't build docs for implementations of virtual libraries

Fixes #2138

Signed-off-by: Jon Ludlam <jon@recoil.org>

src/lib.ml

@@ -268,6 +268,7 @@ let modes        t = t.info.modes
 
 let virtual_     t = t.info.virtual_
 
+let implements   t = t.implements
 let src_dir t = t.info.src_dir
 let orig_src_dir t = Option.value ~default:t.info.src_dir t.info.orig_src_dir
 let obj_dir t = t.info.obj_dir

src/lib.mli

@@ -39,6 +39,7 @@ val main_module_name : t -> Module.Name.t option Or_exn.t
 val wrapped : t -> Wrapped.t option Or_exn.t
 
 val virtual_ : t -> Lib_modules.t Lib_info.Source.t option
+val implements : t -> t Or_exn.t option
 
 val special_builtin_support
   : t -> Dune_file.Library.Special_builtin_support.t option

src/odoc.ml

@@ -287,7 +287,9 @@ let setup_toplevel_index_rule sctx =
 let libs_of_pkg sctx ~pkg =
   match Package.Name.Map.find (SC.libs_by_package sctx) pkg with
   | None -> Lib.Set.empty
-  | Some (_, libs) -> libs
+  | Some (_, libs) ->
+    Lib.Set.filter ~f:(fun lib ->
+      Option.is_none (Lib.implements lib)) libs
 
 let load_all_odoc_rules_pkg sctx ~pkg =
   let pkg_libs = libs_of_pkg sctx ~pkg in