vendor_remote_repository generated alias targets don't work

When set `render_config(default_alias_rule = "opt")` for `vendor_remote_repository`. The generated alias rules are not working with the following error:

```
ERROR: .../BUILD.bazel:254:12: error loading package '@@crate_index//': cannot load '@@crate_index//:alias_rules.bzl': no such file and referenced by '//...'
```

A closer look showed that the generated `alias_rules.bzl` is not included in the generated `crate_index` repo.

If I understood correctly, `alias_rules.bzl` contains no dynamically generated content, and could be directly referenced from `rules_rust`.