Apply a big hammer for stale pre-OpenZFS files

[emaste]

Summary

-DNO_CLEAN builds have had trouble across the OpenZFS import. It's not worth the effort to try to address this with any granularity; instead, just trigger on a .depend file indicating a tree from before the import, and remove the whole cddl object tree.

[kevans]

Yeah, I think this should sufficiently whack the necessary bits.

[mmacy]

Thanks