fix(index): Don't talk to crates.io when using alt registry

[epage]

Ideally we'd find the proper registry and talk to it but that takes a
lot more work to support.
Until then, we'll just act like the registry has no packages in it and
put in some specific hacks to support it.

This should be less brittle than our previous approach which just hoped
we sprinkled config.registr().is_none() in the right places.

Fixes #732

[bobrik]

I have publish = ["myregistry"] in my Cargo.toml and cargo-release still fails with this change.

Printing pkg.config.registry() reveals that it's None. Is it expected that I also need to pass --registry=myregistry?

[epage]

At this time; yes. Feel free to open an issue about respecting public = [names]. Not many of us have experience with alternative registries to know how all of these pieces fit together.

[bobrik]

Sure, I made #736.