Making your CSS modular is a difficult thing to do, but using Webpack makes this so much easier. By adding a single line to your Webpack config, you can require you CSS, Stylus, Less, etc. files right from your directives and keep everything together.
What's the point of excluding
node_modules from the style loader? It means that you can't require
You're absolutely correct. :-)
Are there no significant performance hits by bundling the CSS within the bundle.js? Are you sacrificing the browser's inherent caching of external CSS files?
If you are concerned, there are ways that you can inform the loaders to save the result in a separate file. But I would recommend that before adding that layer of complexity, you first measure whether it will make a difference for your use case.
How would you make your .styl code use variables from other files with webpack? For example, I have a file variables.styl located elsewhere, how can I use it in kcd-hello.styl?
You can still do normal imports in your stylus files. Treat them like normal stylus files and you'll be able to import variables normally :-)