Don't use default systems as hight level input / default

[blaggacao]

This just doesn't correspond to the granularity requirement and not all outputs are derivations.

Push the definition closer to the derivations:

getPlatforms = d: if d?meta && d.meta?platforms
              then d.meta.platforms
              else throw "gotta put systems in place";
    smartPlatforms = lib.attrsets.mapAttrs (system: d:
        lib.filterAttrs (_: v: builtins.any (x: x == system) (getPlatforms v)) d
    );

/cc @tomberek

[blaggacao]

Here, we have an interpretation of this: https://github.com/divnix/hive/blob/main/pasteurize.nix#L129

[blaggacao]

Maybe we'll revise this one good day.