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.

tests/derivations.scm

@@ -831,10 +831,10 @@
                     (derivation-prerequisites-to-build store drv))
                    ((build* download*)
                     (derivation-prerequisites-to-build store drv
-                                                       #:substitutable?
+                                                       #:substitutable-info
                                                        (const #f))))
         (and (null? build)
-             (equal? download (list output))
+             (equal? (map substitutable-path download) (list output))
              (null? download*)
              (null? build*))))))
 
@@ -879,7 +879,7 @@
           ;; See <http://bugs.gnu.org/18747>.
           (and (null? build)
                (match download
-                 (((? string? item))
+                 (((= substitutable-path item))
                   (string=? item (derivation->output-path drv))))))))))
 
 (test-assert "derivation-prerequisites-to-build in 'check' mode"