This is a followup to 18fa433bf5 and
<https://issues.guix.gnu.org/43508>.
* guix/packages.scm (derivation=?, input=?): New procedures.
(bag->derivation, bag->cross-derivation): Add calls to
'delete-duplicates'.
* tests/packages.scm ("package-derivation, inputs deduplicated"): New
test.