--unpublished flag doesn't work with private registries.

[emilycf-cw]

The --unpublished flag only skips crates which are published in the crates.io registry. If I am instead using a private registry, none of my crates will be published there, so this flag does nothing.

[scullionw]

i'm having the same issue

[scullionw]

@epage is this expected behaviour?

[epage]

It is not. In general, custom registry support is weak as I don't have any scenarios I can test it on.

Minimum Steps

• Convert registry names to registry URLs using this cargo-edit code
• Correctly initialize crates_index::Index from the correct URL

Next-level steps

• Mirror cargo in determining the default registry that should be used for publish

Related #368