Backport Fix #364 for good: remove magical Rhino top-level package objects.

Instead of hard-coding a list of known problematic top-level
package names, we detect what needs to be removed by listing
the properties in the Packages object.

Backport: In 0.5.x, the LiveConnect classes were removed before
loading env.js (unlike 0.6.x). The backport leaves these steps in the
order they were in 0.5.x.

(cherry picked from commit 856029644aa01c16b2df0c7e696615e956b55eec)

Conflicts:

	sbt-plugin/src/main/scala/scala/scalajs/sbtplugin/env/rhino/RhinoJSEnv.scala