ChecksumChecker validates if a task is up to date by calculating its source files checksum.

NoneChecker is a no-op Checker.

TimestampChecker checks if any source change compared with the generated files, using file modifications timestamps.

SourcesCheckable defines any type that can check if the sources of a task are up-to-date.

StatusCheckable defines any type that can check if the status of a task is up-to-date.