[Pre-RFC] `cargo package` should include fewer files by default

Vorpal:

That should block publishing in my opinion, just as it would for normal dependencies.

That would break publishing of serde. serde_json depends on serde, but serde depends on serde_json for doc tests. And serde_derive depends on serde for tests too (which in turn depends on serde_derive). At work we had a similar problem with a project that we wanted to publish to crates.io. Making use of this dev-dependency removal feature allowed us to unblock publishing.