packages: Allow inputs to be plain package lists.
* guix/packages.scm (add-input-label, sanitize-inputs): New procedures.
(<package>)[inputs, propagated-inputs, native-inputs]: Add 'sanitize' property.
* doc/guix.texi (Defining Packages, package Reference):
(Defining Package Variants): Adjust examples accordingly.
* tests/packages.scm ("transaction-upgrade-entry, zero upgrades, propagated inputs")
("transaction-upgrade-entry, grafts")
("package-transitive-inputs")
("package-transitive-supported-systems")
("package-closure")
("supported-package?")
("package-derivation, inputs deduplicated")
("package-transitive-native-search-paths")
("package-grafts, indirect grafts")
("package-grafts, indirect grafts, propagated inputs")
("package-grafts, same replacement twice")
("package-grafts, dependency on several outputs")
("replacement also grafted")
("package->bag, sensitivity to %current-target-system")
("package->bag, propagated inputs")
("package->bag, sensitivity to %current-system")
("package-input-rewriting/spec, identity")
("package-input-rewriting, identity"): Use the label-less input style.
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