Test sites plugins (example from the manual)

  $ cat > dune-project <<EOF
  > (lang dune 3.8)
  > (using dune_site 0.1)
  > (name app)
  > 
  > (package
  >  (name app)
  >  (sites (lib plugins)))
  > EOF

  $ cat > dune <<EOF
  > (executable
  >  (public_name app)
  >  (modules sites app)
  >  (libraries app.register dune-site dune-site.plugins))
  > 
  > (library
  >  (public_name app.register)
  >  (name registration)
  >  (modules registration))
  > 
  > (generate_sites_module
  > (module sites)
  > (plugins (app plugins)))
  > EOF

  $ cat > registration.ml <<EOF
  > let todo : (unit -> unit) Queue.t = Queue.create ()
  > EOF

  $ cat > app.ml <<EOF
  > (* load all the available plugins *)
  > let () = Sites.Plugins.Plugins.load_all ()
  > 
  > let () = print_endline "Main app starts..."
  > (* Execute the code registered by the plugins *)
  > let () = Queue.iter (fun f -> f ()) Registration.todo
  > EOF


  $ mkdir plugin
  $ cat > plugin/dune-project <<EOF
  > (lang dune 3.8)
  > (using dune_site 0.1)
  > 
  > (generate_opam_files true)
  > 
  > (package
  >  (name plugin1))
  > EOF

  $ cat > plugin/dune <<EOF
  > (library
  >  (public_name plugin1.plugin1_impl)
  >  (name plugin1_impl)
  >  (modules plugin1_impl)
  >  (libraries app.register))
  > 
  > (plugin
  >  (name plugin1)
  >  (libraries plugin1.plugin1_impl)
  >  (site (app plugins)))
  > EOF

  $ cat > plugin/plugin1_impl.ml <<EOF
  > let () =
  > print_endline "Registration of Plugin1";
  > Queue.add (fun () -> print_endline "Plugin1 is doing something...") Registration.todo
  > EOF

  $ dune build @all 2>&1 | dune_cmd sanitize
  $ dune exec ./app.exe
  Registration of Plugin1
  Main app starts...
  Plugin1 is doing something...