Splits everything that depends on a `pkgs` instance into an optional
attrs, allowing `helpers.nix` to be bootstrapped without `pkgs`.
This required some refactoring:
- `modules.specialArgs` is only available when `pkgs` is used
- `modules.specialArgsWith` now requires `defaultPkgs` be provided
- `builders.*` now have `*With` variants that take `pkgs` as an argument
and a `withPkgs` function that returns the old interface
- Had to define the fixed part of `builders` outside the attrs for now,
to avoid infinite recursion.
- The old `builders` are now deprecated, and print a warning when
evaluated
- `withOptoinalFns` was introduced to merge the optional attrs into the
final lib.
