Change package names to scala.scalajs.{js,compiler,sbtplugin}.

This was agreed upon as final package names for Scala.js at
the Scala core meeting.

In user code, what changes is that you typically will want to
add the following import:

import scala.scalajs.js

so that js.Any et al. are available in scope.
Change organization to "org.scala-lang.modules.scalajs".

This was agreed upon as final organization for Scala.js at
the Scala core meeting.

In user projects, you must change your project/build.sbt
(or wherever you define sbt plugins) and update the
organization. So you should have a line with:

addSbtPlugin("org.scala-lang.modules.scalajs" %
    "scalajs-sbt-plugin" % "0.1-SNAPSHOT")