Support --offline builds

[Qix-]

Could be done one of two ways:

• cargo vendor and a CI script to check if the dependencies match the lockfiles
• Forks + submodules, then using CI to automatically pull new versions when needed
• Combination of the two.

Regardless of if we want to go the vendoring route or submodule route (I'm leaning toward the latter), this is still blocked on rust-lang/wg-cargo-std-aware#23.