Hey folks,

Lately I've been wanting to version control the list of packages that I
install in my user profile so that I can sync it amongst many machines.
So, I took a stab at adding a new '--apply' option to 'guix package'
that reads in a package list from a Scheme file and creates a new
generation of the profile with only those packages are installed.
Here's an example configuration:

    (use-modules (gnu))
    (use-package-modules base less guile emacs admin ruby mail pumpio man)
    
    (list ruby
          coreutils
          less
          man-db
          notmuch
          guile-2.0
          emacs
          dmd
          offlineimap
          pumpa)

Below is a naive patch that does the job, but is unideal because it
doesn't do some nice things like display the diff between generations
before building.  I'm looking for some guidance to make this option mesh
better with the rest of the 'guix package' utility.  Any help is
appreciated.