Expose all local packages as new module argument.

Fixes #81 (closed).

This exposes all our local packages as a new NixOS module argument ourpkgs, which has attributes for all our local packages.

There are a couple of other ways we could expose this:

• We could add ourpkgs to pkgs, either as an attribute, or like an overlay (which would expose the attributes of ourpkgs on pkgs).
• We could expose it as a new config setting (as suggested here).

However, this feels like it has the nicest interface to consuming it. In any case, it is likely a minimal change to switch to either of the other methods.

While I am touching the package definitions, I also switched ourpkgs.zkapissuer to directly expose the executable package.