pub fn can_publish_to(&self, registry: impl AsRef<str>) -> bool {
        let registry = registry.as_ref();
        match self {
            PackagePublish::Unrestricted => true,
            PackagePublish::Registries(registries) => registries.iter().any(|r| r == registry),
        }
    }
Returns true if a package can be published to the given package registry.

# Examples

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

// Unrestricted means this package can be published to any registry.
assert!(PackagePublish::Unrestricted.can_publish_to(PackagePublish::CRATES_IO));
assert!(PackagePublish::Unrestricted.can_publish_to("my-registry"));

// Publish to specific registries but not others.
let crates_io = &[PackagePublish::CRATES_IO.to_owned()];
let crates_io_publish = PackagePublish::Registries(crates_io);
assert!(crates_io_publish.can_publish_to(PackagePublish::CRATES_IO));
assert!(!crates_io_publish.can_publish_to("my-registry"));

// Cannot publish to any registries.
assert!(!PackagePublish::Registries(&[]).can_publish_to(PackagePublish::CRATES_IO));
```
guppy/src/graph/graph_impl.rs