Speed up getDepsScript.nix

[Chris Warburton]

This can take a LONG time, and it seems to spend that time sitting idle,
or spending a chunk of CPU on a bash process.

This looks like a prime opportunity to optimise by porting to
Haskell. Since it calls the GetDeps binary, we might as well move it
into a new binary in the same project, e.g. GetAllDeps or something.

It's basically just JSON manipulation, so aeson (and maybe lenses?)
would do the job.