Broadly speaking, the following things are included:
* there is now a uniform `args` struct that is passed to all
derivations, package headers have been changed appropriately
* overrides are now loaded from a separate `override` folder just
using read-tree.nix
* third-party packages have moved into the `third_party` attribute set
Adds a shell script that supports a subset of the 'pass' interface for
compatibility with kontemplate, and wraps kontemplate in a script that
places this version on the PATH.
This makes it possible to use Cloud KMS encrypted secrets with kontemplate.