Replaced the `isDocs` specialArg added in #1807 with an internal module
option.
We should only use `specialArgs` when absolutely necessary, a module or
`_module.args` is preferred, but those aren't available until `config`
is evaluated so they can't be used for `imports`.
The main problem with `specialArgs` is it makes our modules less
portable.
Luckily that shouldn't be an issue for these context flags.
Let's simplify things by defining all modules in `./plugins`, `./modules`
and `./wrappers/modules`.
Instead of currying `pkgs` into a bootstrapping module, we can require
`defaultPkgs` be provided as a special arg.
This refactor allows us to completely remove `flake-modules/modules.nix`!
