Change the mach-nix provider default to wheel and drop lots of special cases

This also adds a work-around for zfec#50 (fixed, unreleased) which may not be strictly necessary but I don't know when the next zfec will release will happen and it doesn't hurt so let's go with it.

The motivation for this was that I tried to bring a patch in from Twisted and I was having a lot of trouble. Ultimately the problem was just that I was doing it wrong but working through these changes helped me correct my understanding of some pieces of mach-nix and I think the result is better. Even though the line count is about the same, the new lines are mostly comments rather than packaging definitions. Additionally, all of the special cases should really be looked at for every Tahoe-LAFS release and every pypi db bump ... which is a lot of work that it would be nice to avoid.

This also adds a bit of structure where I should be able to place the Twisted patch and have it work without further fuss (as demonstrated by the zfec patch that this adds).