Cryptoless Tahoe-LAFS storage server plugin

* 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

Cannot figure out how to have a VCS or other non-package-name dependency w/ install_requires.

Use pip-tools to manage dependencies.