Drastically simplify distribution building for 2.11.

See README and individual commits for info.

The main idea is that these scripts should do nothing more than repackaging org.scala-lang % scala-dist and its transitive dependencies. No duplication allowed.

Review by @jsuereth, /cc @retronym, @gkossakowski.

This has all been tested locally. I'll do a dry run on jenkins tomorrow and link to the results.