[Bug][http-clien-js]: no api layer being exported for unbranded emitter output

### Describe the bug

there's no api layer being exported for unbranded emitter output, but we have models layer
```
"exports": {
".": "./dist/index.js",
"./models": "./dist/models/index.js"
}
```
wonder if that's expected? Also, should we leverage tshy for the build giving cjs doesn't support subpath export.

### Reproduction

N/A

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.