patch 8.2.4264: Vim9: can use old style autoload function name

Problem:    Vim9: can use old style autoload function name.
Solution:   Give an error for old style autoload function name.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index d6142a6..a3f28f1 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -3078,7 +3078,7 @@
 
   var lines =<< trim END
       vim9script noclear
-      def script#autoloaded()
+      export def Autoloaded()
       enddef
       def Broken()
         var x: any = ''
@@ -3091,7 +3091,7 @@
   lines =<< trim END
       vim9script
       def CallAutoloaded()
-        script#autoloaded()
+        script#Autoloaded()
       enddef
 
       function Legacy()
@@ -3196,7 +3196,7 @@
 
   let lines =<< trim END
     vim9script
-    def script#func()
+    export def Func()
     enddef
   END
   call mkdir('Xdir/autoload', 'p')
@@ -3206,7 +3206,7 @@
       vim9script
       set cpo+=M
       exe 'set rtp^=' .. getcwd() .. '/Xdir'
-      au CmdlineEnter : ++once timer_start(0, (_) => script#func())
+      au CmdlineEnter : ++once timer_start(0, (_) => script#Func())
       setline(1, 'some text')
   END
   call writefile(lines, 'XTest_redraw_cpo')