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.
Moved `extraFiles` from `modules/output.nix` into its own file `modules/files.nix`.
Users should now assign text to a `text` attribute, however they could
also assign a file path to a `source` attribute instead.
The old method of directly assigning a string still works, and is
coerced to the new type along with a deprecation warning.
