pub fn is_never(&self) -> bool {
        match self {
            PackagePublish::Unrestricted => false,
            PackagePublish::Registries(registries) => registries.is_empty(),
        }
    }
Returns true if a package cannot be published to any registries.

# Examples

```
use guppy::graph::PackagePublish;

assert!(!PackagePublish::Unrestricted.is_never());
assert!(!PackagePublish::Registries(&[PackagePublish::CRATES_IO.to_owned()]).is_never());
assert!(PackagePublish::Registries(&[]).is_never());
```
guppy/src/graph/graph_impl.rs