Adds a dependency from 'targets' to 'sources'

        Both 'targets' and 'sources' can be either list
        of target names, or a single target name.

def add_dependency (self, targets, sources):
        """Adds a dependency from 'targets' to 'sources'

        Both 'targets' and 'sources' can be either list
        of target names, or a single target name.
        """
        if isinstance (targets, str):
            targets = [targets]
        if isinstance (sources, str):
            sources = [sources]
        assert is_iterable(targets)
        assert is_iterable(sources)

        for target in targets:
            for source in sources:
                self.do_add_dependency (target, source)