**Problem**
managedScalaInstance := false fails Java-only tests because
the test runner is based on a Scala Runner and sbt 2.x
currently derives the ScalaInstance only from update.
**Solution**
For the purpose of Java-only testing, create a dummy Scala Instance.
For the details about this PR, please see the blog post https://eed3si9n.com/sbt-remote-cache/.
* Add cache basics
* Refactor Attributed to use StringAttributeMap, which is Map[StringAttributeKey, String]
* Implement disk cache
* Rename Package to Pkg
* Virtualize packageBin
* Use HashedVirtualFileRef for packageBin
* Virtualize compile task