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.
This introduces a new Nix derivation that, given an attribute set of
layer hashes mapped to store paths, will create a layer tarball for
each of the store paths.
This is going to be used by the builder to create layers that are not
present in the cache.
Relates to #50.
Simplifies the wrapper script used to invoke Nix builds from Nixery to
just contain the essentials, since the layer grouping logic is moving
into the server itself.
Introduces three new types representing each of the possible package
sources and moves the logic for specifying the package source to the
server.
Concrete changes:
* Determining whether a specified git reference is a commit vs. a
branch/tag is now done in the server, and is done more precisely by
using a regular expression.
* Package sources now have a new `CacheKey` function which can be used
to retrieve a key under which a build manifest can be cached *if*
the package source is not a moving target (i.e. a full git commit
hash of either nixpkgs or a private repository).
This function is not yet used.
* Users *must* now specify a package source, Nixery no longer defaults
to anything and will fail to launch if no source is configured.
Some upcoming changes might require the Nix build to be split into
multiple separate nix-build invocations of different expressions, thus
splitting this out is useful.
It also fixes an issue where `build-image/default.nix` might be called
in an environment where no Nix channels are configured.
Instead of requiring the server component to be made aware of the
location of the Nix builder via environment variables, this commit
introduces a wrapper script for the builder that can simply exist on
the builders $PATH.
This is one step towards a slightly nicer out-of-the-box experience
when using `nix-build -A nixery-bin`.
