While importing those modules from the host system is valid, it is often
a mistake that introduces non-reproducibility. This patch prints a
warning when that happens.
* guix/gexp.scm (gexp-attribute): Add #:validate parameter and honor it.
(gexp-modules)[validate-modules]: New procedure.
Pass it to 'gexp-attribute'.
* tests/gexp.scm ("gexp-modules, warning"): New test.