Adds a new kind of API node, MkModuleObject to represent a module/class object, (MkModuleInstance is coming in a future PR).

This also refactors the generation of subclass edge so that subclass edges target the MkModuleObject node. The use predicate associates uses with a MkModuleObject. (This is similar to how MkClassInstance works in JS)

This is a mostly semantics-preserving change, except we now perform a trackUseNode step to the subclass edge generation.

Also contains a bugfix in DataFlow::ModuleNode.getAnImmediateReference that was needed to associate MkModuleObject to its uses.

Evaluation shows good performance and 3 new sinks (which are due to the addition of trackUseNode mentioned above).