Docs.settings: mappings under the api/ directory.

All dependencies of scala-dist under the javadoc classifier are included as
"api/jars/$artifactId-$version-javadoc.jar".

The core jars are also expanded under api/scala-{library|reflect|compiler}.

Also make constructing mappings reusable.

Needed to use updateClassifiers to get -javadoc artifacts,
don't seem to need to include the javadoc classifier in the scala-dist config.