channels: Refer to 'guile-json-3'.

Fixes a regression introduced in 84af1e74.

* guix/channels.scm (whole-package-for-legacy): Refer to GUILE-JSON-3,
not GUILE-JSON.

guix/channels.scm

@@ -505,7 +505,7 @@ (define lib
                    ;; In the "old style", %SELF-BUILD-FILE would simply return a
                    ;; derivation that builds modules.  We have to infer what the
                    ;; dependencies of these modules were.
-                   (list guile-json guile-git guile-bytestructures
+                   (list guile-json-3 guile-git guile-bytestructures
                          (ssh -> guile-ssh) (tls -> gnutls)))))
 
 (define (old-style-guix? drv)