Newer packages can have explicit dependencies for their Setup.hs
scripts. This feature allows us to supply setup dependencies for
packages that do not specify explicit dependencies. This makes it
possible to implement policies like having older custom Setup.hs scripts
depend on an older Cabal lib (e.g. adding a default dep on Cabal < 1.23)

The nix-local-build branch implements this policy and it goes further
and tracks all setup deps so that no "ambient" packages are ever used.