Support for removed modules in Scala 2.12

[lrytz]

In Scala 2.12:

• The Scala project no longer depends on the scala-actors-migration, scala-continuations-plugin and scala-continuations-library modules
• The Scala project no longer builds the scala-actors module

This is implemented in this PR: https://github.com/scala/scala/pull/4466

Running a community build on that PR fails (https://jenkins-dbuild.typesafe.com:8499/job/Community-2.11.x-manual/163/consoleFull):

[scala:error] java.lang.RuntimeException: Cannot find artifacts dir for: org.scala-lang#scala-actors

ScalaBuildSystem.scala needs to be updated. @cunei, could you take a look?

[jsuereth]

@lrytz We're still taking care of the bintray migration. Which means, I'd estimate ~ 7 days before we can get to this. If you have time to make the PRs to bintray, we can help guide a litlte bit, but we're a bit swamped right now. I don't want to delay 2.12.0-M1, but I think that's what you may have to do here.

[cunei]

I've done some debugging: dbuild is not at fault here. In the root of the Scala source tree there is a file called "dbuild-meta.json". When using the ant build script, dbuild is unable to extract automatically the dependencies of the Scala compiler and the list of generated artifacts; the file "dbuild-meta.json", therefore, is used to provide this information to dbuild. The file must be kept in sync with the ant build file, however. I submitted a pull request against the 2.12.x branch: https://github.com/scala/scala/pull/4507