Previously this function didn't actually have a lot of effect. If a
build A had a dependency B, Hydra would start B first. But on the
next scan through the queue, it would start A anyway, because of the
"busy => 0" restriction.
Now the queue runner won't start a build if a dependency is already
running. (This is not necessarily optimal, since the build may have
other dependencies that don't correspond to a build in the queue but
could run. One day we'll start all Hydra builds in parallel...)
Also, for performance, use computeFSClosure instead of "nix-store
-qR". And don't bother with topological sorting because it didn't
have an effect anyway since the database returns dependencies in
arbitrary order.
906b129f6a