* guix/derivations.scm (substitution-oracle): Use
'substitution-path-info' instead of 'substitution-paths'. Turn SUBST
into a vhash from path to <substitutable>. Change the returned
procedure to provide a <substitutable> instead of a Boolean.
* tests/derivations.scm ("substitution-oracle and #:substitute? #f"):
Mock 'substitutable-path-info' instead of 'substitutable-paths'.