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.

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

@@ -360,6 +360,31 @@
      </para>
     </listitem>
    </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>