css-loader@4 broke @import statements referencing files with
extensions other than .css, unless those @import statements are
compiled away by another loader. Upstream is more interested in
arguing that such @import statements are semantically incorrect than
applying the one line fix.
https://github.com/webpack-contrib/css-loader/issues/1164
Signed-off-by: Anders Kaseorg <anders@zulip.com>