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.

Co-authored-by: Antonio Nuno Monteiro <anmonteiro@gmail.com>
Signed-off-by: Samuel Hym <samuel.hym@rustyne.lautre.net>

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

@@ -1,11 +1,22 @@
 open Dune_site.Private_
 module Data = Dune_site_plugins_data
 
-let readdir dirs =
-  List.concat
-    (List.map
-       (fun dir -> Array.to_list (Sys.readdir dir))
-       (List.filter Sys.file_exists dirs))
+let meta_fn = "META"
+
+let readdir =
+  let ( / ) = Filename.concat in
+  let readdir_noexn dir =
+    try Sys.readdir dir with
+    | Sys_error _ -> [||]
+  in
+  fun dirs ->
+    List.concat
+      (List.map
+         (fun dir ->
+           List.filter
+             (fun entry -> Sys.file_exists (dir / entry / meta_fn))
+             (Array.to_list (readdir_noexn dir)))
+         dirs)
 ;;
 
 let rec lookup dirs file =
@@ -208,8 +219,6 @@ 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

otherlibs/dune-site/test/opam-uninstall.t

@@ -92,6 +92,5 @@ issue by recreating this empty directory.
   $ mkdir -p _install/lib/app/plugins/plugin1
 
   $ OCAMLPATH=_install/lib:$OCAMLPATH _install/bin/app
-  Fatal error: exception The plugin "plugin1" can't be found in the search paths "$TESTCASE_ROOT/_install/lib/app/plugins".
-  [2]
+  Main app starts...