Improvements on unused argument detection in { foo, bar }@args

[Aleksanaa]

Is your feature request related to a problem? Please describe.
NixOS/nixpkgs#331085

Describe the solution you'd like
Don't report unused argument when using together with @args pattern, but do report unused argument if they have a default value in this case (the default value won't take effect)

nix-repl> foo = { a ? 1 }@args: args

nix-repl> bar = foo {}               

nix-repl> bar
{ }

Here a should be reported to be unused (maybe add a new error type)

Additional context
NixOS/nixpkgs#331085
NixOS/nixpkgs#330589

[inclyc]

Maybe split these two improvements to two issues. The first one seems to be a bug, and bugs usually have higher priority than feature requests.