refactor: implement a validation system for builders / translators, reorganize files

[yusdacra]

TODOs:

• actually implement the validation functions (or leave it for later PR)
• update documentation
• comment out the code?

This reorganizes translators, builders, discoverers and adds validation for these. These are also made extensible through the dream2nix config.

Example:

dream2nix.lib.init {
  # declare extras from a nix file
  # the file should have a nix function that takes {dlib, lib, config} as arg
  # and should return an attrset like below
  config.extra = ./nix/d2n/extras.nix;
  # declare extras as an attrset
  config.extra = {
    subsystems = {
      # add new modules
      ruby.discoverers.default = ./nix/d2n/ruby/discoverer.nix;
      ruby.translators.bundix = ./nix/d2n/ruby/bundix.nix;
      # existing modules can be overrided
      rust.builders.crane = ./nix/d2n/rust/crane.nix;
    };
    # add new fetchers
    fetchers.ipfs = ./nix/d2n/fetchers/ipfs.nix;
    fetchers.gitea = ./nix/d2n/fetchers/gitea.nix;
    # existing fetchers can be overrided
    fetchers.http = ./nix/d2n/fetchers/http-proxied.nix;
  };
  # compose multiple extras
  # note: .dream2nixExtras is a hypothetical standardized output
  config.extra = [
    haskellSubsystemFlake.dream2nixExtras
    crystalSubsystemFlake.dream2nixExtras
    gleamSubsystemFlake.dream2nixExtras
  ];
}

If modules are to be used, this could be able to easily modified to use lib.evalModules for validation and evaluating. Personally I'd prefer using something like https://github.com/divnix/yants for validation and keep the code currently as is.

Supersedes #151

[jaen]

Well, that's certainly nicer than my shit attempts xD

I think it would be useful to get this in even without validation or even if the eventual plan is to go full on modules like #156 suggests, just for the external extensibility it affords. So a +1 on incorporating this, maybe even with postponing actual validation to a follow-up PR – would be nice to base my Ruby (and other) stuff on actual upstream extensibility implementation.

[DavHau]

It seems like many files have been moved and edited but git doesn't show them as moved files, instead files are deleted and re-created. This breaks the git history of these files.
I'm not sure if this is easy to fix now, retrospectively.
Next time please do the move and the modify in separate commits, so git won't get confused.

I still think we would benefit a lot by just using evalModules from nixpkgs/nixos/lib/eval-config-minimal.nix. But I will keep this discussion in #156 as it might explode the scope here.

[DavHau]

Thanks a lot!