Deny publishing packages with non-standard name formats (Fix #6043)

[brahmlower]

This PR resolves #6043.

The issue requesting this feature mentions adding a warning, but because packages can't be removed once published, it seemed to make more sense to outright deny the publishing attempt by default. Packages with non-snake-case names can still be published using the allow-non-snake-name option.

[rust-highfive]

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[brahmlower]

I had a brief chance to look into the failing test case last night. The resolve::passes_validation test is failing because test packages are created with non-conformant names, which are now blocked by the changes in this PR. I haven't been able to reproduce the issue locally though. It looks like there's a CI environment variable that might be playing a factor here, but I'm not too sure...

[alexcrichton]

Thanks for the PR! In light of https://internals.rust-lang.org/t/cargo-and-rustup-development/8615/1 I don't think we're willing to merge this at this time. This is a relatively large change to Cargo itself which I unfortunately think we don't have tie to prioritize and review until after the edition and planning.

[brahmlower]

Gotcha- that definitely makes sense given that context. Thanks for letting me know!