[Bug]: Plugins is not exported in beta.48

### Reproduction link or steps

`"./plugins": "./dist/plugins-index.mjs"` is missing from `package.json`, cannot import `replacePlugin` from `rolldown/plugins`

### What is expected?

Import OK

### What is actually happening?

Package path ./plugins is not exported from package

### System Info

```Shell
"rolldown": "1.0.0-beta.48"
```

### Any additional comments?

I'm using `pnpm` pkg manager