Precisamos incluir no paths o caminho para o webpack achar o arquivo.
common.js
const paths = {
root: join(__dirname, '..'),
src: join(__dirname, '..', 'src'),
dist: join(__dirname, '..', 'dist'),
normalizeCss: join(__dirname, '..', 'node_modules', 'normalize.css')
}
E atualizamos o caminho no cssLoader
cssLoader: {
test: /\.css$/,
include: [paths.src, paths.normalizeCss, paths.highlight],
use: ['style-loader', 'css-loader']
},