What I did
I upgraded the postcss-loader dependency to v4.2.0, which allows users to configure their postcss plugins with a postcss.config.js file. However, unlike previous attempts at updating postcss dependencies, I kept the postcss version at 7.0.35 by adding it as a peerDep of postcss-loader (which is now required).
This should supersede #8907 because the postcss-loader does the searching and also combines the config passed to the loader with the config file found.
I also removed the ident option on the config because postcss-loader release notes said it isn't needed anymore.