* guix/gexp.scm (<gexp-input>)[gexp-input]: Rename to...
[%gexp-input]: ... this. Adjust callers accordingly.
(gexp-input): New procedure.
(gexp-inputs)[add-reference-inputs]: When the input is a list, check
whether each item is already 'gexp-input?' and to not rewrap those.
(gexp-outputs)[add-reference-output]: Likewise.
(gexp->sexp): Likewise.
* tests/gexp.scm ("input list splicing + gexp-input +
ungexp-native-splicing"): New test.