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). This change has been introduced in
1f3f1cb4e8e05de3913813e2d8747ed6601fc2fa in the 0.6.x branch due to
issues with env.js. The backport changes the order of these steps in
the 0.5.x branch as well.

(cherry picked from commit 856029644aa01c16b2df0c7e696615e956b55eec)

Conflicts:

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