This can result in unexpected behavior, so explicitly erroring is best.
We can of course add support for this later if we see real value in it
and can come up with predictable rules for how it should work.
This is too complex to easily support; better to explicitly error for
now vs. the current behavior which is just silently doing something
other than you probably expect.
This change tells Webpack (for those using it) that the config file
used should be tracked as a dependency and that the CSS should be
re-compiled if that config file changes.
It's careful to make sure the config file is loaded fresh every single
time to avoid weird caching issues.
