Allow options with type "package" to be store paths

For example, this allows writing

  nix.package = /nix/store/786mlvhd17xvcp2r4jmmay6jj4wj6b7f-nix-1.10pre4206_896428c;

Also, document types.package in the manual.

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

@@ -106,6 +106,15 @@ options = {
     </listitem>
   </varlistentry>
 
+  <varlistentry>
+    <term><varname>types.package</varname></term>
+    <listitem>
+      <para>A derivation (such as <literal>pkgs.hello</literal>) or a
+      store path (such as
+      <filename>/nix/store/1ifi1cfbfs5iajmvwgrbmrnrw3a147h9-hello-2.10</filename>).</para>
+    </listitem>
+  </varlistentry>
+
   <varlistentry>
     <term><varname>types.listOf</varname> <replaceable>t</replaceable></term>
     <listitem>
@@ -138,4 +147,4 @@ You can also create new types using the function
 <varname>mkOptionType</varname>.  See
 <filename>lib/types.nix</filename> in Nixpkgs for details.</para>
 
-</section>
+</section>