Fall back to our internal packaging if it is not supplied.

Also, shift towards less use of overlays.  Explicit arguments are better when
possible because they don't induce freaky action at a distance.

eg: nix-build --arg trialArgs '["-x"]'

* switch from tahoe-lafs src on my laptop to github
* package required updated versions of tahoe-lafs dependencies
* use overlays to avoid version skew between different python libs
* copy/paste/edit the nixpkgs expressions instead of overrideAttrs because the
  latter doesn't converge, I guess?

Not currently working