‘guix package’ spring cleanup

[Ludovic Courtès]

With the recent commit series, scripts/package.scm is much less ugly
than before.  In particular, generic code to manipulate profiles,
generations, etc. has been moved to (guix profiles).  Hopefully that
will make it easier to extend it, to write unit tests, and to implement
other user interfaces.

Feedback welcome!

Thanks,
Ludo’.

PS: Yes it’s spring, in the Southern hemisphere.