patch 8.2.4216: Vim9: cannot use a function from an autoload import directly

Problem:    Vim9: cannot use a function from an autoload import directly.
Solution:   Add the AUTOLOAD instruction to figure out at runtime.
            (closes #9620)
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index 506b0ee..7878788 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -703,6 +703,41 @@
   set opfunc=
   bwipe!
   delete('Xdir', 'rf')
+  nunmap <F3>
+  &rtp = save_rtp
+enddef
+
+def Test_set_opfunc_to_autoload_func_directly()
+  mkdir('Xdir/autoload', 'p')
+  var save_rtp = &rtp
+  exe 'set rtp^=' .. getcwd() .. '/Xdir'
+
+  var lines =<< trim END
+      vim9script
+      export def Opfunc(..._)
+        g:opfunc_called = 'yes'
+      enddef
+  END
+  writefile(lines, 'Xdir/autoload/opfunc.vim')
+
+  new
+  lines =<< trim END
+      vim9script
+      import autoload 'opfunc.vim'
+      nnoremap <expr> <F3> TheFunc()
+      def TheFunc(): string
+        &operatorfunc = opfunc.Opfunc
+        return 'g@'
+      enddef
+      feedkeys("\<F3>l", 'xt')
+      assert_equal('yes', g:opfunc_called)
+  END
+  CheckScriptSuccess(lines)
+
+  set opfunc=
+  bwipe!
+  delete('Xdir', 'rf')
+  nunmap <F3>
   &rtp = save_rtp
 enddef