modules: * export breaks when trying to export variables from imported modules

[megane]

This is a variation of #843.

I have attached a possible patch for this.

;; Works
(module
 bar
 *
 (import chicken scheme)
 (define b 2))

(module
 foo
 ()
 (import chicken scheme)
 (import bar)
 (export b)
 
 )
(import foo)
(print b)

;; Doesn't work
(module
 bar2
 *
 (import chicken scheme)
 (define b2 2))

(module
 foo2
 *
 (import chicken scheme)
 (import bar2)
 (export b2)
 
 )

(import foo2)
(print b2) ;; <- Error: unbound variable: b2