Suggested by Julien Lepiller and myglc2
at <https://lists.gnu.org/archive/html/guix-devel/2017-11/msg00106.html>.

* gnu.scm (%try-use-modules): New procedure.
(package-module-hint, service-module-hint): New procedures.
(try-use-modules): New macro.
(use-package-modules, use-service-modules, use-system-modules): Use it.
* tests/guix-system.sh: Test it.