The useDependenciesExclusive in the SetupWrapper controls two bits of
behaviour: if the deps given for building the Setup.hs are the only
ones allowed, and if we should make version cpp macros available when
building the Setup.hs. Currently we either want both behaviours or
neither behaviour. We want them in the case that a package declares a
custom setup with explicit dependencies.

It makes sense however to have one behaviour without the other. In
particular it makes sense if we want to implement a policy where we
supply default dependencies for older packages that have custom setup
scripts but don't specify any deps. In this case we want to make the
default deps exclusive, but we don't want to use the version macros,
because those ought to be reserved for packages that are doing the right
thing and opting-in to the new world of explicitly specified setup deps.

So this patch splits the behaviour into two controls, but still uses
both together so there's no change in behaviour yet.