With this change, the Spotless Gradle plugin will leverage more of the
core Gradle functionality for incremental builds, thus avoiding some
of the complication present in the previous implementation.

- SpotlessTask runs the formatter for each source file, and generates the
  formatted file into a designated output directory (if format is required)
   - If the task is not incremental, all prior outputs are removed
   - For each added file, the formatter is run, and the output is
     written if different from the original
   - For each removed file, the previous output is removed, if present
- SpotlessCheck takes as input the output directory of the SpotlessTask,
  and will fail if any outputs are present
- SpotlessApply will copy any formatted outputs back over the original source

This mechanism has a number of benefits and makes things a lot simpler:
- The spotless cache is not required
- SpotlessTask is now a simple incremental task and benefits from the
  usual up-to-date checking. It should also support being made cacheable.

At this stage, SpotlessCheck does not produce informative error messages
on failure.