lib/types: Introduce lazyAttrsOf

The standard attrsOf is strict in its *values*, meaning it's impossible to
access only one attribute value without evaluating all others as well.
lazyAttrsOf is a version that doesn't have that problem, at the expense
of conditional definitions not properly working anymore.

lib/types.nix

···
       functor = (defaultFunctor name) // { wrapped = elemType; };
     };
 
+    # A version of attrsOf that's lazy in its values at the expense of
+    # conditional definitions not working properly. E.g. defining a value with
+    # `foo.attr = mkIf false 10`, then `foo ? attr == true`, whereas with
+    # attrsOf it would correctly be `false`. Accessing `foo.attr` would throw an
+    # error that it's not defined. Use only if conditional definitions don't make sense.
+    lazyAttrsOf = elemType: mkOptionType rec {
+      name = "lazyAttrsOf";
+      description = "lazy attribute set of ${elemType.description}s";
+      check = isAttrs;
+      merge = loc: defs:
+        zipAttrsWith (name: defs:
+          let merged = mergeDefinitions (loc ++ [name]) elemType defs;
+          # mergedValue will trigger an appropriate error when accessed
+          in merged.optionalValue.value or elemType.emptyValue.value or merged.mergedValue
+        )
+        # Push down position info.
+        (map (def: mapAttrs (n: v: { inherit (def) file; value = v; }) def.value) defs);
+      emptyValue = { value = {}; };
+      getSubOptions = prefix: elemType.getSubOptions (prefix ++ ["<name>"]);
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: lazyAttrsOf (elemType.substSubModules m);
+      functor = (defaultFunctor name) // { wrapped = elemType; };
+    };
+
     # List or attribute set of ...
     loaOf = elemType:
       let

nixos/doc/manual/development/option-types.xml

···
    </varlistentry>
    <varlistentry>
     <term>
+     <varname>types.lazyAttrsOf</varname> <replaceable>t</replaceable>
+    </term>
+    <listitem>
+     <para>
+      An attribute set of where all the values are of
+      <replaceable>t</replaceable> type. Multiple definitions result in the
+      joined attribute set. This is the lazy version of <varname>types.attrsOf
+      </varname>, allowing attributes to depend on each other.
+      <warning><para>
+       This version does not fully support conditional definitions! With an
+       option <varname>foo</varname> of this type and a definition
+       <literal>foo.attr = lib.mkIf false 10</literal>, evaluating
+       <literal>foo ? attr</literal> will return <literal>true</literal>
+       even though it should be false. Accessing the value will then throw
+       an error. For types <replaceable>t</replaceable> that have an
+       <literal>emptyValue</literal> defined, that value will be returned
+       instead of throwing an error. So if the type of <literal>foo.attr</literal>
+       was <literal>lazyAttrsOf (nullOr int)</literal>, <literal>null</literal>
+       would be returned instead for the same <literal>mkIf false</literal> definition.
+      </para></warning>
+     </para>
+    </listitem>
+   </varlistentry>
+   <varlistentry>
+    <term>
      <varname>types.loaOf</varname> <replaceable>t</replaceable>
     </term>
     <listitem>