ScalaModule#docJar doesn't work if scala version doesn't line up with ScalaWorker

[lihaoyi-databricks]

Gives a Scala signature has wrong version error. This is because the scaladoc version we're using is from the scala version of ZincWorker, not the scala version of your module. We should create a separate classloader to hold the scaladoc classpath so we can ensure it lines up with that of your module