Towards minimal build for publishing core to maven.

Use `unless` attribute in targets so that `docs.skip` influences dependency graph.

Create `scaladoc` task in `pack.core` so that we don't need `pack.done`
for publishing the core.

Move source bundle creation from `dist.src` to `osgi.core`/`osgi.done`.

Split dependencies of maven publishing into core/all variants:
`osgi.core` creates osgi bundles for just lib/reflect/comp.
Targets for 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`.