Created by: joshhunt
Fixes https://github.com/facebook/create-react-app/issues/7919 and fixes https://github.com/facebook/create-react-app/issues/7777
An extra loader to preprocessed CSS files was introduced in https://github.com/facebook/create-react-app/commit/914c95e87facb7ca5debf5bd5514f211807f71bf#diff-dc0c4e7c623b73660da1809fc60cf6ba, but the importLoaders option was not updated to account for this.
This meant that when a stylesheet was imported from another stylesheet (like when using . @import as reported in https://github.com/facebook/create-react-app/issues/7777, or composes: reported in https://github.com/facebook/create-react-app/issues/7919), Webpack would not run all required loaders over the imported stylesheets. As the importLoaders value was still 2, it would run postcss-loader and resolve-url-loader but most importantly, it incorrectly skipped the preProcessor,
I fixed the issue here, and added a small comment warning future developers about this, as seen elsewhere in the webpack config.
I verified this change by running it over my reproduction case mention in https://github.com/facebook/create-react-app/issues/7919.