This commit adds some logic to the modules normalization process to
allow including modules under a user-defined namespace. It achieves it
by:
- flattening any attribute sets in `includes` that are non-empty nor
  contain any of a module's valid attributes (`lib.modules.VALID_KEYS`).
- Erroring out on dupplicate namespaces.
- Mapping namespaced includes do normal modules declaring an option
  `${namespace}` of the include as a submodule.

This allows specifying includes in a module like:
```
{
  includes.mynamespace0 = ./mymodule0.nix;
  includes.mynamespace1 = ./mymodule1.nix;
}
```

and is approximatively desugared by `lib.modules.normalize` into:
```
{
  includes = [
    { options.mynamespace0 = lib.submodule mymodule0.nix; }
    { options.mynamespace1 = lib.submodule mymodule1.nix; }
  ];
}
```

This was inspired by nixpkgs' `lib.modules.doRename`.

@@ -169,0 +177,4 @@
+              if
+                builtins.isAttrs include
+                && include != { }
+                && !builtins.any (n: builtins.elem n lib.modules.VALID_KEYS) (builtins.attrNames include)

@@ -169,0 +204,4 @@
+              in
+              builtins.foldl' filter [ ] flattened;
+
+            doNamespace =

@@ -169,0 +219,4 @@
+                      description = "include ${namespace}";
+                    };
+                  };
+                  # config.${namespace} = lib.modules.override 1000 {};

@@ -169,0 +222,4 @@
+                  # config.${namespace} = lib.modules.override 1000 {};
+                };
+          in
+          builtins.seq throwOnConflict builtins.map doNamespace flattened;

@@ -172,3 +230,2 @@
         __file__ = builtins.toString module.__file__ or file;
-        __key__ = builtins.toString module.__key__ or key;
-        includes = module.includes or [];
+        includes = normalizeIncludes module.includes or [ ];