cargo: Generate a Cargo.lock if one wasn't found

先阅读现有的 cargo 插件以及 issue 42 的链接规范。跟踪 track、fetch 和 stage 流程，然后验证此处描述的 generate_lock 模式、默认行为和错误情况；当 Cargo.lock 的生成和依赖重建遵循所述规则时，即表示完成。

Spec from https://gitlab.com/BuildStream/bst-plugins-experimental/-/issues/42

Will behave like this:

• track: Cargo.lock will be generated by the host's cargo IF (generate_lock is true) OR (generate_lock is 'if-missing' and Cargo.lock doesn't exist). If generate_lock is false and Cargo.lock doesn't exist, raise an error. Then the existing logic for tracking cargo sources will happen. The ref will now encode everything necessary to reconstruct the vendored libraries, so the generated Cargo.lock is discarded
• fetch: The sources are fetched according to the ref, as they are now
• stage: The sources are staged according to the ref, as they are now
• Buildstream will default to generate_lock: false, and projects can (if they wish) globally override that default to generate_lock: if-missing to make the cargo plugin work in all scenarios. Then generate_lock: true can be used if Cargo.lock needs to be overwritten/ignored for whatever reason (i.e. a hypothetical repo that hasn't been updated in a while and Cargo.lock specifies dependencies w/ known security issues, so you want to force it to use newer libraries)
• If a project doesn't want to grow a dependency on cargo from the host to track, it can keep generate_lock: false as the default, and then maintain its own Cargo.lock files & stage them before the cargo source:

sources:
- kind: git_tag
  url: whatever:foobar.git
- kind: local
  path: files/cargo/foobar/Cargo.lock
- kind: cargo
  generate_lock: false # <-- from project defaults