You can import files relative to your project’s root (resolving node_modules/ from the root folder) by prefixing with a tilde ~:
@import '~react-select/dist/react-datetime.css';
This is a poorly documented Webpack (a redundant phrase) convention, see https://github.com/webpack-contrib/css-loader/issues/12#issuecomment-41940311 and What does a `~` tilde in a CSS `url()` do?