Allow a custom visibility so long as at least one version is public

Summary:
The `visibility` config is limited to public packages because that's the
only place it makes sense. But it's possible for a package to have one
version public and other versions private (by virtue of being transitive
deps) - like in [this post][1].

[1]: https://fb.workplace.com/groups/rust.language/posts/27084881544467104

Reviewed By: diliop

Differential Revision: D63963530

fbshipit-source-id: 4a04ebbb355fa8e8f44ea6d041bba48dc5cfa62d

src/fixups.rs

@@ -126,7 +126,8 @@ impl<'meta> Fixups<'meta> {
             fixup
         };
 
-        if fixup_config.custom_visibility.is_some() && !index.is_public_package(package) {
+        if fixup_config.custom_visibility.is_some() && !index.is_public_package_name(&package.name)
+        {
             return Err(anyhow!(
                 "only public packages can have a fixup `visibility`."
             ))