how to keep original directory structure？


```sh
src
  - tool1
    - tool1.ts
  - tool2
    - tool2.ts
  - tool3
    - tool3.ts
  - index.ts
```
Such as  above, `src/index.ts` is `exports` all, then use `tsup src/index.ts --format esm,cjs,iife` build,it will output:

```sh
dist
  - index.mjs
  - index.global.js
  - index.js
```
it not I want.