Change relSourceMap and absSourceMap into a single mapSourceURI option

Also create proper source maps for overridden sources (fixes #758)
No. `scalaJSOpts` is the state object used to construct the options and at the same time implements the `ScalaJSOptions` trait (which is exposed to the phases and does not have `relSourceMap`/`absSourceMap`). If this is should not be the state of the option construction, it may not contain vars but must be a var itself (which is also acceptable).