Fix PostCSS dependency message handling (#73529)
Closes PACK-3606
Closes https://github.com/vercel/next.js/issues/73422
It's `"dependency"`, not `"file-dependency"`... (the Typescript types
wouldn't have caught that either though)
https://github.com/webpack-contrib/postcss-loader/blob/8024e67c7b57c3656063f50465d924a24ec796ce/src/index.js#L191-L219
Apparently, we don't have a test for that...
