Proposal: merge `packages` and `settings` submodules

[srid]

Context: https://nixos.zulipchat.com/#narrow/stream/413949-haskell-flake/topic/.E2.9C.94.20.60custom.60.20setting.20stopped.20working/near/430086733

Instead of separate packages.<name>.* and settings.<name>.*, why not have a single submodule namespace? Isn't that simpler DX?

viz.: Instead of doing:

{
  packages = {
    aeson.source = "1.5.0";
    myhaskell.source = ./haskell;
  };
  settings = {
    hello_rust.custom =self'.packages.hello_rust;
    myhaskell.libraryProfiling = true;
  };
}

why not just:

{
  packages = {
    hello_rust.source = self'.packages.hello_rust
    myhaskell.source = ./haskell;
    myhaskell.setting.libraryProfiling = true;
    aeson.source = "1.5.0";
  };
}

?