Hadrian: trace the execution of expensive Cabal calls
We use Cabal to parse, configure, register and copy packages, which are
expensive operations that are currently not visible to Shake's profiling
infrastructure. By using traced we tell Shake to add these IO actions
to the profiling report, helping us to identify performance bottlenecks.
We use short tracing keys, as recommended in Shake docs: the name of the
current target is already available in the rest of the profiling