less mutation, interface without scala dependencies

  - Initialize FileManager in one go, instead of with lots of mutations.
  - To allow calling partest reflectively from sbt's test interface,
    make signature of AntRunner pure Java.
sbt's partest test-interface reflectively calls sbtrunner
clean up classpath handling

  - remove mutation, code duplication
  - allow versioned jars for scala-library etc by looking only at prefix
  - specialized lib must come first
  - don't crash when codelib not found (e.g. for stand-alone scala-xml)
  - scalacheck classloader: instantiate framework directly
  - make fileManager lazy to avoid NPE