Use cargo as the dependency resolver

[nc7s]

Hi, this project is of great help in packaging. I find that cargo-tree has been integrated into cargo, and thinking of it, it's the best dependency resolver - the one we actually use!

It may be such a process: deserialize Cargo.toml as cargo::util::toml::TomlManifest, .to_real_manifest() into cargo::core::manifest::Manifest, then cargo::core::resolver::resolve() it into a cargo::core::resolver::Resolve, which has the resolved dependency tree.

Then use the data to find what is outdated, what is not in Debian archive yet, etc.

EDIT: Or, "hijack" cargo tree.

[jamessan]

The problem using the resolved dependencies, as I discovered, is that it's greedy. We really want to work off the relationships described in Cargo.toml, since that gives us the lower bound the project needs.