[racket] show-requires isn't submodule-aware?

Re https://github.com/greghendershott/racket-mode/issues/76

The underlying reason seems to be that `show-requires` from
`macro-debugger/analysis/check-requires` doesn't consider submodules.

Below is a tiny example with 2 source files and 1 checker file.

Is this as-intended, or something I should file a PR about?


;;; mod.rkt
#lang racket
(first '(1)) ;`first' provided by racket/list but not racket/base


;;; submod.rkt
#lang racket
(module+ test
  (first '(1))) ;`first' provided by racket/list but not racket/base


;;; chk.rkt
#lang racket
(require (only-in macro-debugger/analysis/check-requires show-requires))
(show-requires "mod.rkt")
;; => '((bypass racket 0 ((racket/base 0 #f) (racket/list 0 #f))))
(show-requires "submod.rkt")
;; => '((bypass racket 0 ((racket/base 0 #f))))
;; But expected same result as for mod.rkt:
;;    '((bypass racket 0 ((racket/base 0 #f) (racket/list 0 #f))))