[pigment-css][core] Use npm package name for folder name

[oliviertassinari]

Steps to reproduce

#41317 (comment)

Current behavior

No response

Expected behavior

No response

Context

So developers who browse the source have 0 doubts that they are at the right place.

Your environment

npx @mui/envinfo

  Don't forget to mention which browser you used.
  Output from `npx @mui/envinfo` goes here.

Search keywords: folder name

[danilo-leal]

I believe this PR #41354 should be enough to close this issue 👍

[oliviertassinari]

It's a great first step. The examples look correct: https://github.com/mui/material-ui/tree/master/examples/pigment-css-nextjs-ts.

But the package folder names don't match with the npm package name, e.g. https://github.com/mui/material-ui/tree/master/packages/pigment-react. I'm reopening.