Register a mapping of the dependent to resource name.

        After calling, dependency_register.dependents[dependent] should
        contain resource_name.

def _register_dependent(self, dependent, resource_name):
        '''
        Register a mapping of the dependent to resource name.

        After calling, dependency_register.dependents[dependent] should
        contain resource_name.
        '''
        if dependent not in self.dependents:
            self.dependents[dependent] = []
        self.dependents[dependent].insert(0, resource_name)