Compile scalalib overrides with the scalalib, not the library.

This way, their .class files do not pollute scalajs-library.jar.
It also simply makes more sense.

Note that the scala-2.11 branch can merge this by recursive strategy without conflict, see https://github.com/sjrd/scala-js/commit/8dfdf7e5d8c00d4984d1ad11e63c879669513f7a
This is also one step closer to having a unified build with a single branch.