Refactor IdentifierNamespace

org.opendaylight.yangtools.yang.model.api.meta.IdentifierNamespace is an interface specifying a get() method, which is not directly implemented anywhere.

The reality is that this interface is not really meant to be instantiated – there are no yang-model-api-level means to acquire an instance of it, as all lookups operate on the .class object instead.

Since the parser internals will be disconnected from this interface with YANGTOOLS-1204, reconsider the overall design of this class. It certainly needs to lose the get() method and it probably should be turned into an abstract class, whose sole constructor will prevent any attempts at instantiation.