Annotate task with @CacheableTask so that the build cache can be used

[hakanai]

We're doing some investigations on --build-cache and looking at the Gradle profiler's output to see which tasks aren't being cached.

After fixing a few of the cases where our configuration was causing things not to be cacheable, and fixing our own custom checks, forbidden-apis is now one of the slowest tasks which isn't coming from the cache.

It would be nice if this task were cacheable.

Adding the annotation might be the only thing needed, but it might also change the version of Gradle the plugin is built against...

[uschindler]

Hi, at moment this is not possible due to minimum Gradle version. And it is not well tested yet.
At moment we also need outputs (the task has no outputs, so there's nothing to cache.

[uschindler]

Actually this can't be enabled, as the task has no outputs. I let this on TODO list. Once we have a "state" output file, this could be enabled.