Look for META files rather than just directories to list plugins

If the plugin directory contains anything but directories that each
contain a `META` file, loading plugins fails

This can happen naturally in particular in the following scenario:
- install a package providing a plugin to a command in another package
- and remove that plugin package.
This leaves an empty directory which triggers that failure.

otherlibs/dune-site/src/plugins/plugins.ml

@@ -1,10 +1,16 @@
 open Dune_site.Private_
 module Data = Dune_site_plugins_data
 
+let meta_fn = "META"
+
 let readdir dirs =
+  let ( / ) = Filename.concat in
   List.concat
     (List.map
-       (fun dir -> Array.to_list (Sys.readdir dir))
+       (fun dir ->
+         List.filter
+           (fun entry -> Sys.file_exists (dir / entry / meta_fn))
+           (Array.to_list (Sys.readdir dir)))
        (List.filter Sys.file_exists dirs))
 ;;
 
@@ -208,8 +214,6 @@ let load file ~pkg =
   { Meta_parser.name = Some pkg; entries }
 ;;
 
-let meta_fn = "META"
-
 let lookup_and_load_one_dir ~dir ~pkg =
   let meta_file = Filename.concat dir meta_fn in
   if Sys.file_exists meta_file