Allow setting the local P2 cache dir in the Spotless Gradle plugin

[bric3]

This adds a way to control in which directory the Eclipse based formatters will be cached. This PR adds a cacheDirectory method in the config of each Eclipse based steps.

The reason is that, by default the eclipse resolver will place the downloaded artifacts in ~/.m2/repository/dev/equo/p2-data, this folder is a bit alien in Gradle based build. And having it nested in the maven repository structure makes it a bit harder to manage in a CI.

Given it's a Gradle related change, I choose to not add this under the maven plugin since .m2/ is already maven managed.

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[jochenberger]

Maybe the Gradle plug-in could also set a more sensible default?

[jochenberger]

Somewhere inside $GRADLE_USER_HOME for example?

[bric3]

I hesitated to do that, as it's a breaking change.

[jochenberger]

IMHO, if we just break a potentially existing cache, it's not so much of an issue. But maybe you're right. 🤷🏼‍♂️

[bric3]

Actually I slept on it, and I kind of agree with you know, it could be considered a bug.

And as a matter of fact, under the hood spotless uses a "fake" eclipse, and there's a mechanism that is supposedly handling Gradle, but it only works if the GRADLE_USER_HOME env var is there. See

https://github.com/equodev/equo-ide/blob/731353e7046f7f7e73da7fd5524991168bccc045/solstice/src/main/java/dev/equo/solstice/p2/CacheLocations.java#L80-L104

I suppose there are various reasons why it's like that in solstice. But spotless knows where it is run so it should provide sensible defaults. I'm commiting someting in that regard.