Allow dependencies between jobsets

[expipiplus1]

Not entirely sure how this would work. Here's the use case

We have two set of builds, one which builds our haskell packages, and the other which builds an identical set of packages but with profiling. It would be very handy to only build the profiling version of a package if the non-profiled version worked.

I suppose that this could be done already by inserting a dependency in the nix expression, but that's really gross.