Support cargo workspaces

[AyanSinhaMahapatra]

Fixes #3598

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Looked for possible updates in documentation and added updates if applicable
• Updated CHANGELOG.rst (if applicable)

[JonoYang]

@AyanSinhaMahapatra Looks good to me, I left a comment about adding a newline

[pombredanne]

LGTM.... with a nit wrt. using "workspace" as a placeholder value?
Could we use something better, may be using extra data instead for fields that should be copied back from the workspace in a lower package?

Also would the top level workspace exist as a package of its own or not?

[AyanSinhaMahapatra]

Could we use something better, may be using extra data instead for fields that should be copied back from the workspace in a lower package?

Yeah this is much better and correct, doing this now.

Also would the top level workspace exist as a package of its own or not?

So I did consider doing this, but the top-level workspace usually does not have any purl fields there, only some extra data wrt source/licenses, see https://github.com/nexB/scancode-toolkit/blob/support-cargo-workspaces/tests/packagedcode/data/cargo/cargo-with-workspace/Cargo.toml#L12 for example. So for now we are not adding top-level packages here, but we can revisit this if we get better examples. But if there are other manifests of other flavours, we'll be adding packages for those.

[AyanSinhaMahapatra]

I've removed the improved package assembly part from this PR and pushed this as a separate branch at: https://github.com/nexB/scancode-toolkit/tree/update-package-assembly, since this was not ready to merge. Otherwise this was reviewed and approved, so merging!