Allow a no-cargo setup for bzlmod (#2565)
Usage looks like so:
```
crate = use_extension("@rules_rust//crate_universe:extension.bzl", "crate")
crate.spec(package = "anyhow", version = "1.0.77")
....
crate.from_specs(name = "crates")
```
It might make sense to merge the annotation attributes into the spec so
we don't have to duplicate things, but we can probably iterate on this
in the future, this API is still experimental, yeah?
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