patch 8.2.4174: Vim9: can use an autoload name in normal script

Problem:    Vim9: can use an autoload name in normal script.
Solution:   Disallow using an autoload name.
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index 6ecea56..3734709 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -1622,6 +1622,18 @@
   delete('Xdir', 'rf')
 enddef
 
+def Test_autoload_name_wring()
+  var lines =<< trim END
+     vim9script
+     def Xscriptname#Func()
+     enddef
+  END
+  writefile(lines, 'Xscriptname.vim')
+  CheckScriptFailure(lines, 'E1263:')
+
+  delete('Xscriptname')
+enddef
+
 def Test_import_autoload_postponed()
   mkdir('Xdir/autoload', 'p')
   var save_rtp = &rtp