gexp: <lowered-gexp> separates sources from derivation inputs.

gexp: <lowered-gexp> separates sources from derivation inputs.

* guix/gexp.scm (lower-inputs): Return either <derivation-input> records
or store items.
(lower-reference-graphs): Return file/input pairs.
(<lowered-gexp>)[sources]: New field.
(lower-gexp): Adjust accordingly.
(gexp->input-tuple): Remove.
(gexp->derivation)[graphs-file-names]: Handle only the
'derivation-input?' and 'string?' cases.
Pass #:sources to 'raw-derivation'; ensure #:inputs contains only
<derivation-input> records.
* guix/remote.scm (remote-eval): Adjust to the new <lowered-gexp>
interface.
* tests/gexp.scm ("lower-gexp"): Adjust to expect <derivation-input>
records instead of <gexp-input>