* guix/derivations.scm (derivation-input-output-paths): New procedure.
(derivation-prerequisites-to-build): New `use-substitutes?' keyword
argument. Change two return the list of substitutable paths as a
second argument.
* guix/ui.scm (show-what-to-build): Turn `dry-run?' into a keyword
argument. New `use-substitutes?' keyword argument. Use `fold2' and
adjust to use both return values of
`derivation-prerequisites-to-build'. Display what will/would be
downloaded.
* guix/scripts/build.scm (guix-build): Adjust accordingly.
* guix/scripts/package.scm (guix-package): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and
substitutes"): New test.
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