Refactor the module abstraction layer to align more closely to OSGi concepts

Felix' module layer was created as a general modularity layer abstraction which is not specific to OSGi concepts. The main idea for doing this was to create a general modularity layer that could support other modularity approaches. Since Felix' mandate is to create an OSGi implementation, the generic modularity layer actually complicates achieving its goal. In an effort to simplify Felix' implementation, reduce its footprint, and hopefully improve some aspects of performance, the module layer abstractions should be refactored to align with OSGi concepts.