Emacs is currently subtly broken on nixos-unstable, but I don't care
about debugging that.
To work around it, this reintroduces the NixOS stable channel (20.03)
but as a separate attribute set from which attributes like Emacs can be
picked into //third_party.
Externally developed dependencies which have been imported ("vendored") into
this repository. These dependencies come with their own licenses and whatever
else.
Code that is developed inside of this repository, but released to an external
repository via Copybara.