Improvements to the Scala build system

[cunei]

This pull request contains improvements to the Scala build system. In particular:

• It is now possible to rewire (on sufficiently recent versions of Scala) the dependencies that the compiler/library have on the various modules: xml, parser-combinators, partest, scalacheck, scala-swing, etc.
• Also new is the ability to adapt the cross versions of modules to those of previously published modules.- The old options build-target and deploy-target are replaced and subsumed by a new option targets, which is more flexible and general: it is now possible to specify a list of ant targets that should be run, and the directories where they should be run into.
• The system can now automatically use the new ant target publish.local, if present.
• The Scala build system can now use a Scala compiler previously generated in one space in order to compile the compiler into a different space, thereby enabling a bootstrap process.

[jsuereth]

Nice work and flexibility! LGTM once the one very minor comment is taken care of.

[cunei]

Using a for rather than map is mostly a matter of personal style. I changed as you described.