import: json: Consolidate duplicate json-fetch functionality.

* guix/import/json.scm (json-fetch): Return a list or hash table.
  (json-fetch-alist): New procedure.
* guix/import/github.scm (json-fetch*): Remove.
  (latest-released-version): Use json-fetch.
* guix/import/cpan.scm (module->dist-name): Use json-fetch-alist.
  (cpan-fetch): Likewise.
* guix/import/crate.scm (crate-fetch): Likewise.
* guix/import/gem.scm (rubygems-fetch): Likewise.
* guix/import/pypi.scm (pypi-fetch): Likewise.
* guix/import/stackage.scm (stackage-lts-info-fetch): Likewise.

guix/import/cpan.scm

@@ -88,9 +88,10 @@
   "Return the base distribution module for a given module.  E.g. the 'ok'
 module is distributed with 'Test::Simple', so (module->dist-name \"ok\") would
 return \"Test-Simple\""
-  (assoc-ref (json-fetch (string-append "https://fastapi.metacpan.org/v1/module/"
-                                        module
-                                        "?fields=distribution"))
+  (assoc-ref (json-fetch-alist (string-append
+                                "https://fastapi.metacpan.org/v1/module/"
+                                module
+                                "?fields=distribution"))
              "distribution"))
 
 (define (package->upstream-name package)
@@ -113,7 +114,7 @@ return \"Test-Simple\""
   "Return an alist representation of the CPAN metadata for the perl module MODULE,
 or #f on failure.  MODULE should be e.g. \"Test::Script\""
   ;; This API always returns the latest release of the module.
-  (json-fetch (string-append "https://fastapi.metacpan.org/v1/release/" name)))
+  (json-fetch-alist (string-append "https://fastapi.metacpan.org/v1/release/" name)))
 
 (define (cpan-home name)
   (string-append "http://search.cpan.org/dist/" name "/"))