Uses the structured errors feature introduced in the Nix code to
return more sensible errors to clients. For now this is quite limited,
but already a lot better than before:
* packages that could not be found result in 404s
* all other errors result in 500s
This way the registry clients will not attempt to interpret the
returned garbage data/empty response as something useful.
Adds environment variables with which users can configure the package
set source to use. Not setting a source lets Nix default to a recent
NixOS channel (currently nixos-19.03).
Instead of just dispatching on URL regexes, use handlers to split the
routes into registry-related handlers and otherwise(tm).
For now the otherwise(tm) consists of a file server serving the static
directory, rather than just a plain match on the index route.
Previously the code had hardcoded paths to runtime data (the Nix
builder & web files), which have now been moved into configuration
options.
Additionally configuration for the application is now centralised in a
single config struct, an instance of which is passed around the
application.
This makes it possible to implement a wrapper in Nix that will
configure the runtime data locations automatically.
