mg contains mage-specific helpers, such as Deps for declaring
dependent functions, and functions for returning errors with specific error
codes that mage understands.
sh contains helpers for running shell-like commands with an API that’s
easier on the eyes and more helpful than os/exec, including things like
understanding how to expand environment variables in command args.
target contains helpers for performing make-like timestamp comparing
of files. It makes it easy to bail early if this target doesn’t need to be run.