Emit serialized IR files instead of .js + .js.map + .sjsinfo.

The ClasspathBuilder automatically expand .sjsir files into their
desugared VirtualScalaJSClassfile version for now. It is left for
a future commit to get rid of VirtualScalaJSClassfile entirely and
have the packager and optimizer to work directly on the IR file.
Remove VirtualScalaJSClassfile and use VirtualScalaJSIRFile instead.

Now the various tasks working with a ScalaJSClasspath extract the
information they need from the IR directly, and work with that.