Perfecting Rust Packaging

@gus @jauhien @cuviper @alexcrichton et al.

I’m thinking about the requirements for teaching Cargo to allow the local system to override dependencies, and specifically wondering how a packager might extract the library artifacts from a Cargo build in order to put them at the desired location locally (presumably somewhere under /usr/lib). Are you able today to get the precise information out of Cargo to identify which files to copy somewhere? Does Cargo need to do more to make it clear which of its outputs are the packagable artifacts?