Reuse classloaders of the same Scala version

[jvican]

We're potentially classloading the Scala universe twice, one in ScalaInstance and another one in the ClassLoaderCache created internally by ZincUtil.scalaCompiler. We should unify these two so that we construct the classloader cache ourselves, we share it across the two different use sites, and we pass it in explicitly to create AnalyzingCompiler.

[jvican]

This is actually not happening right now, the classloaders in scala instance are not forced because we don't use actualVersion in bloop. But we could use it in the future, so in that case it's important we reuse classloaders accross all the instance.

[jvican]

Now we do use actualVersion. The good thing is that our caching at the scala instance level prevents us from deduplicating classloaders. I'm closing this since it's not necessary anymore.