Move 'check-package-freshness' from 'guix package' to 'packages'.

* guix/scripts/package.scm (%sigint-prompt, call-with-sigint-handler)
  (waiting, ftp-open*, check-package-freshness): Move to...
* gnu/packages.scm: ... here.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>