Disable this test temporarily.

This functionality will be changing, and this test is in the way while preparing for this change.
This test is testing incorrect behaviour, and is superseded by other tests.
Apply new style plugins on top of the core impl classloader and expose nothing to the plugin target.
Simplify ClassLoaderScope.

- Allow attaching of arbitrary classloaders (supports graphs instead of just trees)
- Remove “base” concept
Load “new style” plugins on top of the core classloader, not the plugins classloader.
Move plugin loading integration tests concerned wth class visibility into a separate test class.
Remove unnecessary closure overload method.
Some generic type fixes.