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/userfunc.c b/src/userfunc.c
index 7341878..650dfc9 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -4495,6 +4495,12 @@
 		    }
 		}
 	    }
+	    else if (vim9script && vim_strchr(name, AUTOLOAD_CHAR) != NULL)
+	    {
+		semsg(_(e_using_autoload_name_in_non_autoload_script_str),
+									 name);
+		goto erret;
+	    }
 	}
 	if (var_conflict)
 	{