sourcemap is empty when using multiple entrypoint & esm

Hi. There is sourcemap issue in multiple entrypoint with esm

```
src/
- hello.ts
- index.ts
tsup.config.ts
```

``` ts
// src/index.ts
export * from './hello';
```

``` ts
// src/hello.ts
export function hello() {
console.log('hello');
}
```

``` ts
// tsup.config.ts
import { defineConfig } from 'tsup';

export default defineConfig({
entry: ['src/index.ts', 'src/hello.ts'],
format: ['esm'],
sourcemap: true,
});
```

when it build, both index.mjs.map, hello.mjs.map is empty

``` json
// dist/hello.mjs.map
{"version":3,"sources":[],"sourcesContent":[],"mappings":"","names":[]}

// dist/index.mjs.map
{"version":3,"sources":[],"sourcesContent":[],"mappings":"","names":[]}
```

only chunk file (`chunk-{hashed}`) has sourcemap. maybe chunking has sourcemap issue?

it works correctly with cjs format.

Thanks!