crate_universe: Start auto-populating missing lockfiles and cargo_lockfiles in Bazel 6

https://github.com/bazelbuild/bazel/pull/13417 added the ability to find the workspace root from a `repository_ctx`, and will be in Bazel 6.

Currently, if the `lockfile` or `cargo_lockfile` points at a file which doesn't exist, we give a really unclear error:
```
ERROR: An error occurred during the fetch of repository 'crate_index':
Traceback (most recent call last):
File "/private/var/tmp/_bazel_john/6ebb6fa28311e23a56b6491e5d493861/external/rules_rust/crate_universe/private/crates_repository.bzl", line 34, column 30, in _crates_repository_impl
lockfiles = get_lockfiles(repository_ctx)
File "/private/var/tmp/_bazel_john/6ebb6fa28311e23a56b6491e5d493861/external/rules_rust/crate_universe/private/generate_utils.bzl", line 299, column 36, in get_lockfiles
cargo = repository_ctx.path(repository_ctx.attr.cargo_lockfile),
Error in path: Not a regular file: /Users/john/figma/rules_rust_built/Cargo.Bazel.lock
```

Instead, now that we have this API, we should be able to:
1. Switch `cargo_lockfile` from being an `attr.label` to being an `attr.string`
2. Parse it ourselves, and if it looks like it's in the workspace repo (i.e. starts `@//`, `//`, or `:`), resolve it relative to `repository_ctx.workspace_root`, and use that as the path; otherwise we should fall back to the existing code for remote repositories (though I'm not sure there's a reasonable use-case for using a remote repository for a lockfile).
3. In the resolver rust code, make sure we handle the "does not exist" case by suggesting a repin (and modifying the error message to say the file doesn't exist, and will be created by a repin).
4. In the resolver rust code, make sure the repin will create the file if missing.