packages: <origin> no longer has an 'imported-modules' field.

* guix/packages.scm (<origin>)[imported-modules]: Remove.
(patch-and-repack): Remove #:imported-modules.  Use
'with-imported-modules'.  Remove #:modules argument to
'gexp->derivation'.
(origin->derivation): Adjust accordingly.
* doc/guix.texi (origin Reference): Adjust accordingly.