Repository: create-react-app
Issue: babel-preset-react-app useESModules option no longer transpiling to ESM in version 10.x
Labels: needs triage,issue: bug report

### Describe the bug

Hello everyone,

First of all, I wanted to thank you for all the work that has been made here. We have been using CRA for a long time now and we are happy with it. 

I would try to explain our situation to discover if this is a real bug or it's simply a bad use of the `babel-preset-react-app`. I have made two sandboxes for you. Hope it helps!

We were using `babel-preset-react-app` in our Babel configuration to transpile our React components library. We needed to transpile it to ESModules to enable our CRA-made applications to build with the tree-shaking switch on. That's was working until we upgraded to CRA 4, that's where `babel-preset-react-app` came out not transpiling our files anymore to ESModules.

Here are the sandboxes where the only differences are the package.json dependencies: 

* [babel-preset-react-app 10.x Sandbox](https://codesandbox.io/s/create-react-app-babel-preset-100-8qbnj?file=/package.json:374-583)
```
"dependencies": {
    "@babel/cli": "7.13.16",
    "@babel/core": "7.14.2",
    "@babel/plugin-transform-modules-commonjs": "7.14.0",
    "@babel/runtime": "7.14.0",
    "babel-preset-react-app": "10.0.0"
  },
```

* [babel-preset-react-app 9.x Sandbox](https://codesandbox.io/s/create-react-app-babel-preset-91-0fqcn?file=/package.json:373-580)

```
"dependencies": {
    "@babel/cli": "7.10.4",
    "@babel/core": "7.11.4",
    "@babel/plugin-transform-modules-commonjs": "7.10.4",
    "@babel/runtime": "7.11.2",
    "babel-preset-react-app": "9.1.2"
  },
```

To try these sandboxes, you will have to fork them, after that, you could be able to open a terminal there and run this command `npm run build`. As you can see, the build script inside `package.json` is calling `babel-cli` to build our `/src` folder into `/build`. 

As you will see in the first sandbox(10.x), the files inside `/build` are in CommonJs but in the second one(9.x), the files are in ESModules. Is this a bug? 

We did some digging before posting this issue, we found this interesting [MR](https://github.com/facebook/create-react-app/pull/9374) where we found some light. As I can see this is a bugfix MR. So, we were relying on a bug to transpile our code into ESModules?. We did understand that the optional property `useESModules` in `babel-preset-react-app` was made for this.

Probably we were wrong on that. We know we can achieve this without using `babel-preset-react-app` but we were happily using it because we thought that as with CRA(react-scripts) we would benefit from further updates and features that would come from this project.


### Did you try recovering your dependencies?


Yes

### Environment

Node 12 with the above-listed dependencies.

### Steps to reproduce

Open one of the sandboxes:

1. Fork the sandbox
2. Open a new terminal (right side of the screen, it is a '+' icon)
3. run `npm run build` on the terminal
4. Check `/build` files.

### Expected behavior

We were expecting the first sandbox to transpile the code into ESModules as well as the second one.

### Actual behavior

It is transpiling the code into CommonJS

### Reproducible demo

The sandboxes above.