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.

not sure this fixes anything but it might fix the PrivateStorageio build which
finds autobahn 19.1.1 instead of autobahn 19.8.1.

I think it is abstractly "more correct" too, maybe, if you can convince
yourself there is such a thing.

So we don't have to override them to get a new version anymore.  Also,
upstream has done a better job of integrating them into the overall system
than we have (with respect to other upstream changes).

other approach fails because we cannot upgrade pytest to fix attrs
incompatiblity because pytest has dropped python 2.7 support in 5.x series

Newer typing in nixpkgs requires newer Hypothesis than it has.

This may make no difference but we're not doing anything to override
openssl_1_1 in this overlay so we don't need to get it from `self`.

openssl_1_1 in the pinned version of nixpkgs is 1.1.1c.

* 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