This makes 'guix environment PACKAGE' significantly faster when
substitutes are enabled. Before that, it would lead to many invocations
of 'guix substitute-binary', one per 'derivation-prerequisites-to-build'
call. Now, all these are replaced by a single invocation.
* guix/derivations.scm (derivation-output-paths, substitution-oracle):
New procedures.
(derivation-prerequisites-to-build): Replace #:use-substitutes? with
#:substitutable?. Remove the local 'derivation-output-paths' and
'substitutable?'.
* guix/ui.scm (show-what-to-build): Add 'substitutable?'. Pass it to
'derivation-prerequisites-to-build'.
[built-or-substitutable?]: Use it instead of 'has-substitutes?'.
* tests/derivations.scm ("derivation-prerequisites-to-build and
substitutes"): Use #:substitutable? instead of #:use-substitutes?.
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