imp: soc of eval, modules & options

[blaggacao]

• the reason for this change is to have more transparent separation
  of concern between effectuations of the module system and pre-module
  system effectuations

• with improved flakes support down the line, pre-module system
  effectuations will get more complex

• this also allows to patch the aspects of the evaluation individually while
  tracking other components from upstream. eg. patch options & eval but
  not modules

[zhaofengli]

Currently, this is problematic because eval.nix is embedded into the binary, with its content written into a temporary file when we want to use it. Instead of force-pushing and waiting for GitHub Actions, you can also run all the unit tests with cargo test inside the shell environment (nix-shell / nix develop / direnv).

[blaggacao]

Ah thanks! that's why it fails 😄 I also want to separate them so as to be able to patch the eval.nix logic. thanks for the pointer...

[blaggacao]

Based on this I'll protoype a custom flake-only schema for #71 over at https://github.com/divnix/hive -- which I currently refactor to use colmena.

In such projects, I do think it's even a general advantage to keep the evaluator in local code, for documentation purposes.

[blaggacao]

@zhaofengli this is ready imo 🚀 🎸

[zhaofengli]

As mentioned on Matrix, I'm still not sure whether I'd like the evaluator to be swappable (#82) or not, but this PR looks fine so let's merge it. There are some small nits which I'll fix later. Thanks!