derivations: Rewrite and replace 'derivations-prerequisites-to-build'.

derivations: Rewrite and replace 'derivations-prerequisites-to-build'.

The new 'derivation-build-plan' procedure has a more appropriate
signature: it takes a list of <derivation-inputs> instead of taking one
<derivation>.  Its body is also much simpler.

* guix/derivations.scm (derivation-build-plan): New procedure.
(derivation-prerequisites-to-build): Express in terms of
'derivation-build-plan' and mark as deprecated.
* tests/derivations.scm: Change 'derivation-prerequisites-to-build'
tests to 'derivation-build-plan' and adjust accordingly.