Add a dry-run publish API

[joelgallant]

In rust-lang/cargo#6150, @sfackler suggested that crates.io could have a dry-run publishing API endpoint, so it can enforce any requirements (like wildcard deps) in cargo publish --dry-run.

I'd love to help out with that, if it's something worth doing.

[nkconnor]

Reserved names would also be useful to catch with this; e.g. I got hit with

error: api errors (status 200 OK): cannot upload a crate with a reserved name

trying to publish my first crate, and had to dig pretty hard to find that it was part of the DB migration here

[jtgeibel]

If anyone has time to dig into this, now might be a good time to revisit this. We've been tracking some refactoring in #1522 and this may be easier to implement now. The goal is to make sure the desired checks are done early in the pipeline, before writing anything to the database. All database writes should occur after we return from the --dry-run publish and the only external service I think we should possibly hit is to check team membership if necessary.