Auto merge of #6806 - ehuss:warn-semver-metadata, r=Eh2406

Warn on version req with metadata.

Metadata in a version requirement (such as `1.0.0+1234`) is ignored. This adds a warning that it will be ignored.

On crates.io I found about 5 crates, plus a few dozen google-* crates (presumably all created by the same person) which have dependencies of this form.

See discussion at https://github.com/rust-lang/cargo/issues/6504#issuecomment-451254084. cc rust-lang/crates.io#1059 for ongoing discussion about what to do about publishing such versions.