In 0fb8e4d I forgot that flake input-following exists. I don't _have_ to
ship nixhelm myself, but I do need to include it so I can stop bundling
data for an oudated AT version.
In fact, I need not bundle _anything_ for the AT library to be useful;
it's better to make the builder's `chart` arg mandatory and let the user
supply their own (usually taken from nixhelm). If they get bored of
supplying the chart to every AT instance they can factor it out,
as I have in my own deployment.
In fact, my deployment relies on the AT library, which I used to provide
as a flake output -- but it didn't make sense to have one version of the
turboprop library available inside service definitions and another
available outside. So I've made the whole library a flake output.
This may bite me in the future.
This interface was getting ridiculous.
`app-template` gets released infrequently enough that keeping a static
`chart.nix` is a small tradeoff for the previous lunacy of passing a
whole derivation tree in through the flake.
`mkDerivation` doesn't have to be assigned beforehand; it can just be
passed two attrsets. I only realized this after writing out its
signature in the documentation.
Also, the example flake in the documentation doesn't use nixpkgs,
so I removed it.
