Separating fetching from building for better security

grothesque: > Perhaps, in an ideal world, Cargo could expose a stable API with a more comprehensive internal model of operations and required effects. Our initial plans for an API are plumbing commands. We have a prototype at GitHub - crate-ci/cargo-plumbing: Proposed plumbing commands for cargo · GitHub. The next steps are major refactors of cargo to make it so each command can run in isolation. No one is actively working on this though.