gexp: Add 'lower-gexp' and express 'gexp->derivation' in terms of it.

* guix/gexp.scm (gexp-input-thing, gexp-input-output)
(gexp-input-native?): Export.
(lower-inputs): Return <gexp-input> records instead of tuples.
(lower-reference-graphs): Adjust accordingly.
(<lowered-gexp>): New record type.
(lower-gexp, gexp-input->tuple): New procedure.
(gexp->derivation)[%modules]: Remove.
[requested-graft?]: New variable.
[add-modules]: New procedure.
Rewrite in terms of 'lower-gexp'.
(gexp-inputs): Add TODO comment.
* tests/gexp.scm ("lower-gexp"): New test.