gexp: Move the package and origin compilers to (guix packages).

From now own, (guix packages) depends on (guix gexps); it was the other
way around now.  This means that (guix packages) code can use gexps.

* guix/gexp.scm (origin-compiler, package-compiler): Remove.
  (default-guile-derivation): New procedure.
  (gexp->derivation): Use it instead of 'default-guile' +
  'package->derivation'.
* guix/packages.scm (default-guile-derivation): New procedure.
  (package-compiler, origin-compiler): New variables.
* doc/guix.texi (G-Expressions): Mention extensibility.