* guix/scripts/system.scm (references*, topologically-sorted*,
show-what-to-build*): New procedures.
(copy-closure): Turn into a monadic procedure.
(install): Likewise, and adjust parameter list.
(switch-to-system): Likewise.
(system-derivation-for-action, grub.cfg, maybe-build, perform-action):
New procedures.
(guix-system): Use them.