Issue title: transformer-postcss default.fromJSON is not a function
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# 🐛 bug report
I'm attempting to use SCSS/CSS modules with Parcel 2 beta 1 however, as soon as I add a module import, I get the following error:
```
🚨 Build failed.
@parcel/transformer-postcss: _postcss(...).default.fromJSON is not a function
TypeError: _postcss(...).default.fromJSON is not a function
    at Object.transform (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/config-default/node_modules/@parcel/transformer-postcss/lib/PostCSSTransformer.js:175:38)
    at async Transformation.runTransformer (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/core/lib/Transformation.js:830:5)
    at async Transformation.runPipeline (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/core/lib/Transformation.js:585:36)
    at async Transformation.runPipelines (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/core/lib/Transformation.js:413:40)
    at async Transformation.runPipelines (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/core/lib/Transformation.js:444:34)
    at async Transformation.run (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/core/lib/Transformation.js:316:19)
    at async Child.handleRequest (/Users/andrew/code/_pff/euclid/assets/node_modules/@parcel/workers/lib/child.js:255:9)
✨  Done in 4.78s.

```
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## 🎛 Configuration (.babelrc, package.json, cli command)
.babelrc
```js
{
    "presets": [
        "@babel/preset-env",
        "@babel/preset-typescript",
        ["@babel/preset-react",
          {
            "runtime": "automatic"
          }
        ]
    ]
}
```


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## 🤔 Expected Behavior
The module is correctly imported and compiled.
<!--- Tell us what should happen -->

## 😯 Current Behavior
The build fails
<!--- Tell us what happens instead of the expected behavior -->

<!--- If you are seeing an error, please include the full error message and stack trace -->

## 💁 Possible Solution

<!--- Not obligatory, but suggest a fix/reason for the bug -->

## 🔦 Context
This is a React app with Typescript and I can't use Parcel 2 with css modules. If change it from a module to a plain CSS import, it works fine.
<!--- How has this issue affected you? What are you trying to accomplish? -->

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## 💻 Code Sample
Component.tsx
```
import {FunctionComponent} from "react";
import {NavLink, NavLinkProps} from 'react-router-dom';

import styles from '../../styles/components/SidenavItem.module.scss';
// I've also tried `* as styles`...

export const SidenavItem: FunctionComponent<Pick<NavLinkProps, "to">> = (props) => {
  return (
    <NavLink {...props} style={styles.a}>
      {props.children}
    </NavLink>
  )
};
```

Style.module.scss
```
a {
  color: red;
}
```
<!-- Please provide a code repository, gist, code snippet or sample files to reproduce the issue -->

## 🌍 Your Environment

<!--- Include as many relevant details about the environment you experienced the bug in -->

| Software         | Version(s) |
| ---------------- | ---------- |
| Parcel           | `2.0.0-beta.1`
| Node             | `14.15.1`
| npm/Yarn         | `1.22.10`
| Operating System | MacOS 11.2.2 on a MacBook Air M1

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 Comments: 
 Comment 0: Can you test it with the latest nightly?

Also, we recommend using Parcel's babel presets: https://v2.parceljs.org/languages/babel/#extending-the-default-babel-config
 Comment 1: Just observed the same with `matthewmays@example.org`
 Comment 2: @mischnic thanks for the response 🙂 I just tested with nightly and the babel presets recommended and still got the exact same error.
 Comment 3: This is actually caused by 
```
warning "parcel > @parcel/config-default > @matthewmays@example.org" has unmet peer dependency "postcss@^8.2.1".
```

because you at the moment, you need to add install `postcss` in the project yourself (so that's a workaround). But I admit that this isn't ideal (we have the same problem with `@babel/core`  at the moment)
 Comment 4: Ah, ok thanks for the tip! Feel free to close if you want.
 Comment 5: I am facing similar issue, and adding `matthewmays@example.org` as project dependency didn't help. After some digging, I think I found the issue.

Problem is `transfomer-postcss` depends on 3.2.2 version of `postcss-modules` which depends on `matthewmays@example.org`. 
```
@parcel/transformer-postcss -> postcss-modules@^3.2.2 -> postcss@^7.0.32 
```

Because of this dependency chain, the `postcss` version installed alongside `transform-postcss` is 7.0.32 which does not have `.fromJSON()` method.
 Comment 6: +1 to @smurfpandey. Chanding dependencies to postcss v7 helps us get past this error but then we get to the next one:
```
🚨 Build failed.
@parcel/transformer-postcss: input.toJSON is not a function
```
{'source': 'GithubIssues', 'repo_name_with_owner': 'parcel-bundler/parcel', 'issue_id': 830564152, 'repo_stars': 43245.0}