enh: support Cargo.toml workspace at /

[rbtcollins]

We like our developers to be able to use Cargo directly in parallel with the monorepo build system (because rust-analyzer works better that way still), and so we have /Cargo.toml as a workspace with all our packages and all the third-party crates referenced there.

But I would like to output the buckified BUCK file to third-party/rust/crates/BUCK

Any objections to such a feature?

[cormacrelf]

We have been using a step that copies the [workspace.dependencies] section of the workspace Cargo.toml to a third party dir, adding a dummy [package] and renaming [workspace.dependencies] to [dependencies]. And copying Cargo.lock there. Altogether this simulates having a single cargo package with a bunch of normal dependencies, and we use include_top_level = true with that.

So yeah, that's not ideal, it'd be nice to have this feature.