This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.
This is the first step towards factoring out CLI-based functionality
for Nixery.
Rather than migrating to Bazel, it seems more appropriate to use Nix
for this project.
The project is split into several different components (for data
dependencies and binaries). A derivation for building an image for
Nixery itself will be added.