cargo publish should fail in a workspace

[npmccallum]

It is obvious that cargo publish inside a workspace with multiple crates is not ready for prime time. #1169 details what is required. In particular, attempting to run cargo publish inside a workspace may lead to various problems, including, but not limited to, accidentally publishing crates. While I'm personally excited to see cargo gain the ability to publish a whole workspace, I think cargo should be defensive until the feature is mature.

[npmccallum]

Apparently cargo already does this and I misunderstood the situation.