Expose SubmoduleEffectiveStatement from IncludeEffectiveStatement

yang.model.api.Module exposes its submodules via getModules(). ModuleEffectiveStatement does not have that capability as it inlines all effective statements from submodules.

This is a slight oversight, which we should fix cleanly – which is to expose the included submodule from the 'include' statement which inlines it. We also should be ignoring 'belongs-to' statements for the purposes of inlining and warn/error out if there is a mismatch (i.e. no include for a submodule).