gnu: linux-initrd: Allow Guile modules to be embedded in the initrd.

* gnu/packages/linux-initrd.scm (raw-build-system): New macro.
  (module-package, compiled-module-package): New procedures.
  (expression->initrd): Add `modules' keyword parameter.
  Add "modules" and "modules/compiled" inputs; copy them onto the
  initrd.
* guix/derivations.scm (imported-modules, compiled-modules): Publicize.