Regression: Generating private enum with EnumDiscriminants is no longer possible when the parent enum is public

[yescallop]

The following user code is broken by #377:

#[derive(EnumDiscriminants)]
#[strum_discriminants(name(PrivateDiscriminants), vis(pub(self)))]
pub enum PublicEnum {
    Variant0(bool),
    Variant1 { a: bool },
}

With an error message:

error[E0446]: private type `PrivateDiscriminants` in public interface

This is because the IntoDiscriminant impl of PublicEnum leaks the private PrivateDiscriminants type. Should we maybe exclude the IntoDiscriminant impl when the parent enum is public and the generated enum is private?