Overview of common expression patterns

[fricklerhandwerk]

The ecosystem is messy and people rightfully struggle to make sense of the various conventions for Nix expressions.

Make an overview of commonly encountered patterns and what they mean.

@lf- Thanks for reporting via https://tenforward.social/@noracodes/111926120933093851

[lf-]

cc @NoraCodes

[Qyriad]

This might also be a good place to overview common expression "entry points" and their expectations. nix-build expects a list or set of derivations, nix-shell expects a mkShell family derivation, both of those eval an expression optionally with autocall, but don't perform the package.nix/callPackage pattern, like most files one comes across in Nixpkgs. NixOS modules also have their own rules for the top-level formals and expect an attrset, etc.