Support sharing Cargo crates between bzlmod modules

I want to support bazel for my crates with bzlmod, but, currently (afaik), Cargo dependencies have to be re-added per module, requiring unique naming of extension modules and creating many copies of the same crate. If `crate_universe` could merge all of the used modules' Cargo manifests/specs, I would be able to support Bazel in my crates.

Apologies if this already has been implemented, just let me know.