This was already supported in the internal Scripted used by sbt but not
in the ScriptedPlugin. This is fixed by just copy-pasting the modified
parser. We will have to wait for sbt itself to be built using an sbt
with the upgraded ScriptedPlugin to be able to avoid the code duplication.
To pass File => Unit callback across the classloader boundary
I am encoding it as a java.util.List[File] by overriding
method.
This was needed since Java didn't allow me to cast
from one classloader to the other.