Repo-level aliasing

### Description of the feature request:

I talked to @Wyverald and he said to file a bug here.

At the moment, in rust, it automatically defines the toolchain repos:
```
amd64_to_amd64
amd64_to_arm64
...
```

However, in order to build our host tools, we need to define `host_to_host`. At the moment, this is an exact copy of `amd64_to_amd64` on an amd64 machine, or `arm64_to_arm64` on an arm64 machine.

Because it changes, and because we still want to be able to access it via the original `amd64_to_amd64`, we cannot simply write:
```
use_repo(toolchains, host_to_host = "amd64_to_amd64")
```

In our module extension, I'd like to instead be able to write:
```
def module_extension(module_ctx):
for host in SUPPORTED_PLATFORMS:
for target in SUPPORTED_PLATFORMS:
define_toolchain_repo_rule(name = "{host}_to_{target}".format(host, target), host=host, target=target)
alias_repo(name = "host_to_host", actual = "{host}_to_{host}".format(get_host())
```

### Which category does this issue belong to?

External Dependency

### What underlying problem are you trying to solve with this feature?

Every rule inside the repo rule has to be executed twice, since it isn't a true alias of a repo.

### Which operating system are you running Bazel on?

Linux

### What is the output of `bazel info release`?

6.2.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by reading the module extension example and the existing toolchain repository definitions described in the issue. Investigate how repository aliases are represented and used by use_repo; done means host_to_host can refer to the selected host-specific repository without executing the repository rule twice, while the original repository names remain available.

Written by the indexing model from the issue text.