Nested <defs> don't seem to work

This one needs more research. It appears an icon's internal `<defs>` aren't getting pulled out properly, or are at least causing _something_ to break. In our usage, this hasn't been a huge issue, since I can just inline any of those defs into the icon itself, but it's definitely an annoying step in between a simple export from Sketch/Inkscape/Illustrator and using the icon.