GenerateJavaTask is difficult for gradle to cache

[simongellis-attentive]

Hello!

My company is trying to optimize our gradle caching, and it looks like the DGS GenerateJavaTask is causing a bit of trouble. Specifically, the generateSourcesDir input is a String containing an absolute filepath, which seems to foil remote caching attempts (since repos are cloned to different locations on different machines). Gradle can cache paths well if they're annotated with @PathSensitive(PathSensitivity.RELATIVE), but I'm not sure whether that works on string properties.

I think that PR #356 would allow us to work around this; it'd let us assign specific values to the output dirs, so we could set generatedSourcesDir to some stable empty string and it'd stop busting the cache. Is that PR likely to be merged at some point?