Support non-local Java compiler use in mixed Java/Scala compilation

[lefou]

We need this feature to support SemanticDB generation (#2227) via a Java compiler plugin in mixed Scala/Java projects.

We already introduced non-local Java compilers in #1988. This change now also allows the use of a non-local Java compiler in mixed Java/Scala modules, if any -J option is given.

As compilers are cached, I added the hashCode of the relevant runtime javacOptions to the cache key.

I took the opportunity to remove non-relevant options from the existing Java compiler cache, to reduce the amount of cached compiler instances.

/CC @ckipp01 who implemented #1988.

[lefou]

Currently, we maintain two different caches, one for Scala/Mixed usage, and one for Java-only usage. But technically, the same Java compiler can be used in both setups.

Thus we could avoid holding additional Javac instances in the Scala-compilers-cache, by retrieving the instance from the javac-only-cache first.

[ckipp01]

O nice! It doesn't take much at all to support this. This is great!

[lefou]

Currently, we maintain two different caches, one for Scala/Mixed usage, and one for Java-only usage. But technically, the same Java compiler can be used in both setups.

Thus we could avoid holding additional Javac instances in the Scala-compilers-cache, by retrieving the instance from the javac-only-cache first.

I think we should work on this separately, as it probably involves further refactoring. We currently cache Compilers which is a pair of Scala and Java compile tools, whereas we want to re-use the same JavaTools in multiple Compilers.