This change introduces the cargoLock argument to buildRustPackage,
which can be used in place of cargo{Sha256,Hash} or cargoVendorDir. It
uses the importCargoLock function to build the vendor
directory. Differences compared to cargo{Sha256,Hash}:
- Requires a Cargo.lock file.
- Does not require a Cargo hash.
- Retrieves all dependencies as fixed-output derivations.
This makes buildRustPackage much easier to use as part of a Rust
project, since it does not require updating cargo{Sha256,Hash} for
every change to the lock file.