patch 8.2.4053: Vim9: autoload mechanism doesn't fully work yet

Problem:    Vim9: autoload mechanism doesn't fully work yet.
Solution:   Define functions and variables with their autoload name, add the
            prefix when calling a function, find the variable in the table of
            script variables.
diff --git a/src/userfunc.c b/src/userfunc.c
index ecd2e7c..a3ff38c 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -4080,8 +4080,11 @@
 		eap->skip = TRUE;
 	}
 
-//	if (is_export)
-//	    name = may_prefix_autoload(name);
+	// For "export def FuncName()" in an autoload script the function name
+	// is stored with the legacy autoload name "dir#script#FuncName" so
+	// that it can also be found in legacy script.
+	if (is_export)
+	    name = may_prefix_autoload(name);
     }
 
     // An error in a function call during evaluation of an expression in magic