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/github.scm

@@ -22,31 +22,16 @@
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
-  #:use-module (json)
   #:use-module (guix utils)
   #:use-module ((guix download) #:prefix download:)
   #:use-module (guix import utils)
+  #:use-module (guix import json)
   #:use-module (guix packages)
   #:use-module (guix upstream)
   #:use-module (guix http-client)
   #:use-module (web uri)
   #:export (%github-updater))
 
-(define (json-fetch* url)
-  "Return a representation of the JSON resource URL (a list or hash table), or
-#f if URL returns 403 or 404."
-  (guard (c ((and (http-get-error? c)
-                  (let ((error (http-get-error-code c)))
-                    (or (= 403 error)
-                        (= 404 error))))
-             #f))     ;; "expected" if there is an authentification error (403),
-                      ;; or if package is unknown (404).
-    ;; Note: github.com returns 403 if we omit a 'User-Agent' header.
-    (let* ((port   (http-fetch url))
-           (result (json->scm port)))
-      (close-port port)
-      result)))
-
 (define (find-extension url)
   "Return the extension of the archive e.g. '.tar.gz' given a URL, or
 false if none is recognized"
@@ -144,7 +129,7 @@ the package e.g. 'bedtools2'.  Return #f if there is no releases"
                    "https://api.github.com/repos/"
                    (github-user-slash-repository url)
                    "/releases"))
-         (json (json-fetch*
+         (json (json-fetch
                 (if token
                     (string-append api-url "?access_token=" token)
                     api-url))))