Separate the core SpotlessTask from check and apply

[bigdaz]

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.

[nedtwigg]

Amazing!! Thanks so much. I gave it a careful readthrough, and the only thing left before merging is the changelog and fixing up createIndependentTask. This PR removes SpotlessTask.setApply()/setCheck() which were public methods. So far as I know, the only place that uses createIndependentTask uses it like this, which doesn't make any sense with setApply()/setCheck().

https://github.com/diffplug/blowdryer-diffplug/blob/a9c34895a00c4a7ee2e76db3545aa6a12bd4effa/src/main/resources/spotless/freshmark.gradle#L47-L48

All of these benefits are definitely worth this small breaking change. I also think createIndependentTask was probably a mistake, and could be better implemented by #558 (comment).

I'll do some integration testing with this tomorrow and take a stab at the changelog.

[jbduncan]

Woo! Happy to see this merged in. Thank you very much @bigdaz. 👍

[nedtwigg]

Released in plugin-gradle 4.0.0