The two phases of setupWrapper (configuration and execution) are
separated into the getSetup and runSetup functions. This allows us to
skip configuration if the setup method is known; this is the case when
running multiple commands in sequence or in the new-build
system. Splitting the phases also allows us to choose which _command_ to
run based on the Cabal version in use; setupWrapper demands a fixed
choice of command and only allows the command _flags_ to vary depending
on the version.