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doc: Add Module System chapter start

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doc/manual.xml
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<xi:include href="using/configuration.chapter.xml" />
<xi:include href="using/overlays.chapter.xml" />
<xi:include href="using/overrides.chapter.xml" />
</part>
<part>
<title>Nixpkgs <code>lib</code></title>
<xi:include href="functions.xml" />
<xi:include href="module-system/module-system.chapter.xml" />
</part>
<part xml:id="part-stdenv">
<title>Standard environment</title>

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# Module System {#module-system}
## Introduction {#module-system-introduction}
The module system is a language for handling configuration, implemented as a Nix library.
Compared to plain Nix, it adds documentation, type checking and composition or extensibility.
NOTE: This chapter is new and not complete yet. For a gentle introduction to the module system, in the context of NixOS, see [Writing NixOS Modules](https://nixos.org/manual/nixos/unstable/index.html#sec-writing-modules) in the NixOS manual.
## `lib.evalModules` {#module-system-lib-evalModules}
Evaluate a set of modules. The result is a set with the attributes:
### Parameters {#module-system-lib-evalModules-parameters}
#### `modules` {#module-system-lib-evalModules-param-modules}
A list of modules. These are merged together using various methods to form the final configuration.
#### `specialArgs` {#module-system-lib-evalModules-param-specialArgs}
An attribute set of module arguments that can be used in `imports`.
This is in contrast to `config._module.args`, which is only available within the module fixpoint, which does not exist before all imports are resolved.
#### `specialArgs.class` {#module-system-lib-evalModules-param-specialArgs-class}
If the `class` attribute is set in `specialArgs`, the module system will rejected modules with a different `class`.
This improves the error message that users will encounter when they import an incompatible module that was designed for a different class of configurations.
The `class` value should be in camelcase, and, if applicable, it should match the prefix of the attributes used in (experimental) flakes. Some examples are:
- `nixos`: NixOS modules
- `nixosTest`: modules that constitute a [NixOS VM test](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests)
#### `prefix` {#module-system-lib-evalModules-param-prefix}
A list of strings representing the location at or below which all options are evaluated. This is used by `types.submodule` to improve error reporting and find the implicit `name` module argument.
### Return value {#module-system-lib-evalModules-return-value}
#### `options` {#module-system-lib-evalModules-return-value-options}
The nested set of all option declarations.
#### `config` {#module-system-lib-evalModules-return-value-config}
The nested set of all option values.
#### `type` {#module-system-lib-evalModules-return-value-type}
A module system type representing the module set as a submodule, to be extended by configuration from the containing module set.
This is also available as the module argument `moduleType`.
#### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
A function similar to `evalModules` but building on top of the module set. Its arguments, `modules` and `specialArgs` are added to the existing values.
Using `extendModules` a few times has no performance impact as long as you only reference the final `options` and `config`.
If you do reference multiple `config` (or `options`) from before and after `extendModules`, performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires a new fixpoint to be constructed.
This is also available as a module argument.
#### `_module` {#module-system-lib-evalModules-return-value-_module}
A portion of the configuration tree which is elided from `config`. It contains some values that are mostly internal to the module system implementation.