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.
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.
