[WIP] multiple by-name directories

[quantenzitrone]

Basically a new attempt at #31, but instead of adapting #180, i start from scratch because i wasn't able to rebase it due to lack of codebase knowledge. Also probably lack of skill.

[quantenzitrone]

Problem: the current way of getting AttributeInfo with the overlay does not work for package sets, as one cannot override callPackage in package sets.

makeScope =
  newScope: f:
  let
    self = f self // {
      newScope = scope: newScope (self // scope);
      callPackage = self.newScope { };
      overrideScope = g: makeScope newScope (extends g f);
      packages = f;
    };
  in
  self;