bug#48682: recursive import between (gnu packages chez) and (gnu packages racket)

[Philip McGrath]

Hi, thanks!

> That said… what problem is this fixing raingloom?
>
> Usually, circular top-level references lead things like ‘guix show
> chez-scheme’ (this amounts to loading (gnu packages chez)), but it
> worked fine for me.

It seems like some other commit may have broken the cycle in the mean 
time. Here's what I had been seeing:

```
philip@avalon:~$ guix time-machine 
--commit=ae88e30a0f8403e781f8b01262766cdc46b1018a -- show chez-scheme
Backtrace:
          19 (primitive-load-path "gnu/packages/racket" #<procedure …>)
In gnu/packages/racket.scm:
     22:0 18 (_)
In ice-9/boot-9.scm:
   3409:4 17 (define-module* _ #:filename _ #:pure _ #:version _ # _ …)
  3422:24 16 (_)
   222:29 15 (map1 (((guix licenses) #:select (asl2.0 expat #)) (#) …))
   222:29 14 (map1 (((guix packages)) ((guix download)) ((guix #)) …))
   222:29 13 (map1 (((guix download)) ((guix git-download)) ((# …)) …))
   222:29 12 (map1 (((guix git-download)) ((guix utils)) ((guix …)) …))
   222:29 11 (map1 (((guix utils)) ((guix gexp)) ((guix # gnu)) (#) …))
   222:29 10 (map1 (((guix gexp)) ((guix build-system gnu)) ((# …)) …))
   222:29  9 (map1 (((guix build-system gnu)) ((srfi srfi-1)) ((…)) …))
   222:29  8 (map1 (((srfi srfi-1)) ((ice-9 match)) ((gnu #)) ((…)) …))
   222:29  7 (map1 (((ice-9 match)) ((gnu packages)) ((gnu # #)) # …))
   222:29  6 (map1 (((gnu packages)) ((gnu packages bash)) ((…) …) …))
   222:29  5 (map1 (((gnu packages bash)) ((gnu packages chez) # …) …))
   222:17  4 (map1 (((gnu packages chez) #:select (chez-scheme)) # …))
  3353:12  3 (resolve-interface (gnu packages chez) #:select _ #:hide …)
   260:13  2 (for-each #<procedure 7fe3d526c500 at ice-9/boot-9.scm…> …)
  3360:19  1 (_ _)
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Throw to key `unbound-variable' with args `("resolve-interface" "no 
binding `~A' in module ~A" (chez-scheme (gnu packages chez)) #f)'.
```

I don't think anyone ever figured out where the cycle was. The tl;dr of 
my long comment is that there wasn't a logical cycle yet, and I was 
intending to have (gnu packages racket) import (gnu packages chez), but 
not vice versa, for now.

raingloom had discussed more on IRC---I only happened to see a little of 
it, but I could reproduce.

-Philip