Removed `getClassLoader()` and created new `executeInContext()` that takes a `Callable` becasue we need to make sure the the `CeylonClassLoader` gets properly closed after we're done with whatever needed the loader (ceylon/ceylon-dist#41)
Changed implementation of the `ModuleDescriptorReader` proxy because of changes to the `Launcher` (ceylon/ceylon-dist#41)