[Corepkgs] Have __structuredAttrs on by default

[jonringer]

Really tired of stringly typed bash. Let's make this a bit nicer https://nixos.mayflower.consulting/blog/2020/01/20/structured-attrs/

[blaggacao]

As can be seen from this example, two files—.attrs.sh and .attrs.json—are created if __structuredAttrs is set to true. The JSON file contains all attributes passed to derivation serialised by Nix in the same manner the builtins.toJSON built-in function works; the shell file contains all parameters that are at most one level deep, with Nix lists being converted to Bash arrays and Nix attribute sets to Bash associative arrays. All data nested deeper than one level is only included in the JSON file.

(gist from the article)