check syntax doesn't consider extensions to non-terminals as uses

In the program

#lang racket
(require redex)
(define-language a
  (on ::= 1))
(define-extended-language b a
  (on ::= .... (- 2 1)))

(redex-match? b on (- 2 1))

if i try to fix the typo and rename on to one, I get:

#lang racket
(require redex)
(define-language a
  (one ::= 1))
(define-extended-language b a
  (on ::= .... (- 2 1)))

(redex-match? b one (- 2 1))

where the extension in the language b is still named on.

I would expect that the on in the extended language should be both a usage of on from language a and as a second definition of on when its referred to in the redex-match?

Start with the two Racket examples in the issue and trace the syntax-checking entry point for define-extended-language and non-terminal extensions. The change is done when renaming the base language's non-terminal causes the stale extension reference to be recognized as a use and reported appropriately.