gnu: Add iso-codes variant for use within Guix.

* gnu/packages/iso-codes.scm (iso-codes):
Replace with hidden-package that inherits…
(iso-codes/official): …this ‘new’ variable with its previous value.
* gnu/packages/gtk.scm (gtk+)[inputs]:
Use iso-codes/official to prevent mass rebuilds.

gnu/packages/gtk.scm

@@ -990,7 +990,7 @@ application suites.")
            cups
            graphene
            harfbuzz
-           iso-codes
+           iso-codes/official ;XXX TODO core-updates: use iso-codes
            json-glib-minimal
            libxml2
            rest))

gnu/packages/iso-codes.scm

@@ -1,7 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013 Cyril Roelandt <tipecaml@gmail.com>
 ;;; Copyright © 2016, 2019 Efraim Flashner <efraim@flashner.co.il>
-;;; Copyright © 2018 Tobias Geerinckx-Rice <me@tobias.gr>
+;;; Copyright © 2018, 2022 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -29,7 +29,9 @@
   #:use-module (gnu packages perl)
   #:use-module (gnu packages python))
 
-(define-public iso-codes
+(define-public iso-codes/official
+  ;; This package variant is intended for ‘external’ use, such as users running
+  ;; ‘guix install’, where any deviation from ISO gospel might be harmful.
   (package
     (name "iso-codes")
     (version "4.5.0")
@@ -77,6 +79,25 @@ changes in the ISO standard and will not work with outdated
 information.")
     (license license:gpl2+)))           ; some bits use the lgpl2
 
+(define-public iso-codes
+  ;; This package should be used universally within Guix, e.g., as an input to
+  ;; other Guix packages or in the Guix System installer's country selector.
+  (hidden-package
+   (package
+     (inherit iso-codes/official)
+     (source
+      (origin
+        (inherit (package-source iso-codes/official))
+        (modules '((guix build utils)))
+        (snippet
+         '(begin
+            (substitute* (find-files "." "\\.po$")
+              (("#.*Name for TWN,.*") "")
+              (("^msgid \"Taiwan, .*") "# Guix doesn't use "))
+            (substitute* "data/iso_3166-1.json"
+              (("(Taiwan), [^\"]*" _ name) name))))))
+     (synopsis "Various ISO standards as used by GNU@tie{}Guix"))))
+
 (define-public python-iso639
   (package
     (name "python-iso639")