'List dependencies that are not currently installed.'

@property
def unused_dependency_paths(self):

    unused_dependencies = []
    toolbox_requirements_status = self.toolbox_requirements_status
    for resolver in self._dependency_resolvers:
        if hasattr(resolver, 'unused_dependency_paths'):
            unused_dependencies.extend(resolver.unused_dependency_paths(toolbox_requirements_status))
    return set(unused_dependencies)
