Description:
Command handler package. A command handler takes in a command line, executes
it, and returns a set of implicit inputs/outputs. Handlers can be called by
other handlers.
This is useful for ad-hoc dependency detection. For example, to detect
inputs/outputs when running DMD, we modify the command line so it writes them
to a file which we then read in to determine the inputs/outputs.
If there is no handler, we default to system call tracing.