For instance, it's pointless to keep copy-from-other-stores running if
there are no other stores, or download-using-manifests if there are no
manifests. This also speeds things up because we don't send queries
to those substituters.
Since SubstitutionGoal::finished() in build.cc computes the hash
anyway, we can prevent the inefficiency of computing the hash twice by
letting the substituter tell Nix about the expected hash, which can
then verify it.
again. (After the previous substituter mechanism refactoring I
didn't update the code that obtains the references of substitutable
paths.) This required some refactoring: the substituter programs
are now kept running and receive/respond to info requests via
stdin/stdout.
