linux-initrd: Don't close over core Guile modules.

* nonguix/modules.scm: New file.
* nongnu/system/linux-initrd.scm: Limit 'source-module-closure' to guix and
nonguix modules only.

nonguix/modules.scm

@@ -0,0 +1,27 @@
+;;; Copyright © 2020 Alex Griffin <a@ajgrf.com>
+;;;
+;;; This program is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; This program is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+(define-module (nonguix modules)
+  #:use-module (ice-9 match)
+  #:export (nonguix-module-name?))
+
+(define (nonguix-module-name? name)
+  "Return true if NAME (a list of symbols) denotes a Guix or Nonguix module."
+  (match name
+    (('guix _ ...) #t)
+    (('gnu _ ...) #t)
+    (('nonguix _ ...) #t)
+    (('nongnu _ ...) #t)
+    (_ #f)))