Make spotless tasks all cacheable with the Gradle Build Cache

This should be as simple as adding the @CachableTask annotation to all of the relevant tasks.

[nedtwigg]

An issue that we’re going to have (just popped up in #559) is that most of our FormatterStep cache keys are machine-dependent at the moment. The task reorganization that @bigdaz has proposed is a definite improvement, and will make the local buildcache useful to users. But in order for Spotless to be path-independent (necessary for shared buildcache to be useful), we need to make FileSignature path and machine independent. I think we should discuss and implement this separately, the issue lives at #566.

[nedtwigg]

I did a quick investigation, and it looks like we can probably support this usecase by bumping our minimum-required gradle to 3.0, and we can certainly support it if we bump to 3.5. We have to go all the way to 4.9 to get completely native config-avoidance APIs, which is the other sticking point we’ve had.

No need for a screenshot, a gradle engineer with a real customer usecase is proof-enough for me that there’s value in bumping the minimum. I’m definitely on-board with bumping to 3.0 (3.5 if necessary), and if this heavy-duty usecase can also show that we get measurable benefit from 4.9 then I’m okay with that too. Implementing this is not high on my apocalypse-todo-list, but releasing good PRs quickly is always high on my todo list, even in an apocalypse.