Remove non-existent module from (guix).

* guix.scm (%public-modules): Remove `snix', since that modules doesn't
  exist yet.

Update the (guix) module.

* guix.scm (%public-modules): Update list of current second-level
  modules.

Split (guix) in (guix store) and (guix derivations).

* guix.scm: Move contents to other files.  Just aggregate these.
* guix/derivations.scm, guix/store.scm: New files.

Initial commit.