Repository: material-ui
Issue: [pigment] Exception when importing xxxClasses from @mui/material barrel index
Labels: regression,package: pigment-css,v6.x migration

### Steps to reproduce

Link to live example: https://codesandbox.io/p/devbox/lingering-shape-s9cvl7?file=%2Fsrc%2Fapp%2Fpage.tsx

Steps:
1. Use `styled` from `@mui/material-pigment-css`
2. Try styling component that can be styled with imported MUI classes

```tsx
import { styled } from "@mui/material-pigment-css";
import { Button, buttonClasses } from "@mui/material";

export const StyledButton = styled(Button)(({ theme }) => ({
  [theme.breakpoints.up("md")]: {
    [`& > .${buttonClasses.startIcon}`]: {
      display: "none",
    },
  },
}));
```

### Current behavior

NextJS fails to build and re-throws an exception from pigment

```
 ⨯ unhandledRejection: TypeError: /project/sandbox/src/app/page.tsx: Cannot read properties of undefined (reading 'startIcon')
    at /project/sandbox/src/app/page.tsx:16:153
    at StyledProcessor.processCss (/project/sandbox/node_modules/@pigment-css/react/processors/styled.js:373:59)
    at StyledProcessor.processStyle (/project/sandbox/node_modules/@pigment-css/react/processors/styled.js:306:31)
    at /project/sandbox/node_modules/@pigment-css/react/processors/styled.js:209:12
    at Array.forEach (<anonymous>)
    at StyledProcessor.build (/project/sandbox/node_modules/@pigment-css/react/processors/styled.js:208:20)
    at /project/sandbox/node_modules/@wyw-in-js/transform/lib/plugins/collector.js:26:17
    at /project/sandbox/node_modules/@wyw-in-js/transform/lib/utils/getTagProcessor.js:384:5
    at Array.forEach (<anonymous>)
    at applyProcessors (/project/sandbox/node_modules/@wyw-in-js/transform/lib/utils/getTagProcessor.js:375:10)
```

### Expected behavior

Pigment should be able to successfully build styles using class names just like it does with colors for instance

```tsx
import { red } from "@mui/material/colors";

export const StyledButton = styled(Button)(({ theme }) => ({
  color: red[500], // no error
}));
```

### Context

Migrating to V6 + pigment

### Your environment

<details>
  <summary><code>npx @mui/envinfo</code></summary>

```
System:
    OS: Linux 6.1 Ubuntu 20.04.6 LTS (Focal Fossa)
  Binaries:
    Node: 20.12.1 - /home/codespace/nvm/current/bin/node
    npm: 10.5.0 - /home/codespace/nvm/current/bin/npm
    pnpm: 8.15.6 - /home/codespace/nvm/current/bin/pnpm
  Browsers:
    Chrome: Not Found
  npmPackages:
    @emotion/react:  11.13.3 
    @emotion/styled:  11.13.0 
    @mui/core-downloads-tracker:  6.0.0 
    @mui/icons-material: 6.1.0 => 6.1.0 
    @mui/material: latest => 6.0.0 
    @mui/material-pigment-css: latest => 6.0.0 
    @mui/private-theming:  6.0.0 
    @mui/styled-engine:  6.0.0 
    @mui/system:  6.0.0 
    @mui/types:  7.2.16 
    @mui/utils:  6.0.0 
    @pigment-css/nextjs-plugin: latest => 0.0.20 
    @pigment-css/react:  0.0.20 
    @pigment-css/unplugin:  0.0.20 
    @types/react: latest => 18.3.4 
    react: latest => 18.3.1 
    react-dom: latest => 18.3.1 
    typescript: latest => 5.5.4
```
</details>
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**Search keywords**: pigment, pigment-css, buttonClasses