Before it would list inputs not built, even if the outputs of the given
derivation were already available.
* guix/derivations.scm (derivation-prerequisites-to-build): Add
`outputs' keyword parameter.
[built?, derivation-built?]: New procedures.
[loop]: Add `sub-drvs' parameter. Use `derivation-built?' to check if
the SUB-DRVS of DRV are built before checking its inputs.