Add Babushka::DepCache, fashioned after the caching code in Babushka::Task.
Move the dep logging block to #process!, so #process_with_caching is concerned purely with caching.
Swap the Dep#run_requirements parameters for readability.
Cache dep results with a local DepCache instance instead of on Task.
Remove now-unused caching code from Babushka::Task.
Merge branch 'extract-dep-cache-from-task'