setup: move cffi stuff to mercurial/cffi This patch moves all setup*cffi stuff to mercurial/cffi to make the root directory cleaner. The idea was from mpm [1]: > It seems like we could have a fair amount of cffi definitions, and > cluttering the root directory (or mercurial/) with them is probably not > a great long-term solution. We could probably add a cffi/ directory > under mercurial/ to parallel pure/. [1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-July/086442.html