Auto merge of #6653 - Eh2406:min-pub-dep, r=alexcrichton

Auto merge of #6653 - Eh2406:min-pub-dep, r=alexcrichton

part of the infrastructure for public & private dependencies in the resolver

This is part of my work on public & private dependencies in the resolver from #6129. As discussed there the proptest fuzzers are happy to find exponential blow up with all the back jumping strategies I have tried. So this PR does not have a back jumping strategie nor does it have the proptest fuzzers generating public dependencies. These will both need to change for the feature to stabilize. In the meantime it gives the correct results on the cases it can handle.

With https://github.com/rust-lang/rust/pull/57586 landed there is a lot of work to do on Cargos front end. Adding a UI for this, passing the relevant things to rustc, passing it to crates.io, passing it to cargo-metadata. This is good enough to allow that work to proceed.