Modularize: xml & parser-combinators

[This PR depends on #2851]

These modules move to their own repositories:
  - https://github.com/scala/scala-xml
  - https://github.com/scala/scala-parser-combinators

The modularization depends on the new partest, as the old one's
classpath handling did not support a modularized scala.

The osgi tests take xml and parsers jars from the scaladoc dependencies,
and osgi-fy them, as they are no longer built locally.