derivations: 'derivation-prerequisites-to-build' returns <substitutable>.

* guix/derivations.scm (derivation-prerequisites-to-build): Rename
 #:substitutable? to #:substitutable-info.
[derivation-substitutable?]: Rename to...
[derivation-substitutable-info]: ... this.  Return a list of <substitutable>.
Second return value is now a list of <substitutable> instead of a list
of strings.
* guix/ui.scm (show-what-to-build)[substitutable?]: Rename to...
[substitutable-info]: ... this.
Adjust to new 'derivation-prerequisites-to-build' return value type.
* tests/derivations.scm ("derivation-prerequisites-to-build and
substitutes"): Adjust.
("derivation-prerequisites-to-build and substitutes, local build"):
Likewise.