Rework build to allow publishing core to maven

`ant publish-core-signed-opt-nodocs` creates and publishes the smallest
possible set of jars (library/reflect/compiler) we can send to maven.
It uploads jars with class files and sources, but no docs
(so that we don't need to build scaladoc or any other modules).

`ant publish-core-signed-opt` is similar, but also publishes docs.

For local experimentation: `ant publish-core-local-nodocs`.
With a little bit more work, this can be used instead of `replacestarr`.

This should allow breaking cycles in a principled way in dbuild:
1. build the Scala core (library/reflect/compiler), publish locally
Building the core doesn't have any dependencies on the new modules (it's similar to locker.done).
(Don't test --> no need for partest, skip docs -> ignore soon-to-be-module scaladoc.)
2. Use this Scala core to build all modules (especially partest and scaladoc and all its dependencies)
3. Use partest and scaladoc from step 2. to test and document the core and all modules
4. Publish tested, documented Scala core and modules to maven.

The next step is to build and publish a modularized scaladoc in this build,
and support using a maven-resolved scaladoc to generate docs.

Review by everyone: @gkossakowski, @retronym, @cunei, @jsuereth, @xeno-by, @paulp.