Build scaladoc/scala-interactive like modules.

We're keeping the sources in this repository,
and won't bump their version beyond 1.0.0-RC1 for a while.
Everything else is setup so that the move to their own repository will be seamless.
(And thus can happen post-2.11.0.)

When -Ddocs.skip is set, we also don't build scaladoc.
TODO: allow resolving scaladoc through maven so the above is useful in dbuild