fix caching of observer, project and product

We stored the cache key in the engine and the cache value in the target
object of setupScriptEngineForProduct. This was a result of the "don't
use the global object" refactoring.

Change-Id: Id6796bf60927d14b8968cbb3760d9ca2bcbb7933
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>