make-options-doc: Support Nix-provided declaration links

Previously, the location logic was hardcoded, supporting only
Nixpkgs and NixOps properly, leaving other uses of the module
system without good location support.

(cherry picked from commit cee66a8cd5)

nixos/lib/make-options-doc/options-to-docbook.xsl

@@ -213,6 +213,23 @@
 
   <xsl:template match="attr[@name = 'declarations' or @name = 'definitions']">
     <simplelist>
+      <!--
+        Example:
+          opt.declarations = [ { name = "foo/bar.nix"; url = "https://github.com/....."; } ];
+      -->
+      <xsl:for-each select="list/attrs[attr[@name = 'name']]">
+        <member><filename>
+          <xsl:if test="attr[@name = 'url']">
+            <xsl:attribute name="xlink:href"><xsl:value-of select="attr[@name = 'url']/string/@value"/></xsl:attribute>
+          </xsl:if>
+          <xsl:value-of select="attr[@name = 'name']/string/@value"/>
+        </filename></member>
+      </xsl:for-each>
+
+      <!--
+        When the declarations/definitions are raw strings,
+        fall back to hardcoded location logic, specific to Nixpkgs.
+      -->
       <xsl:for-each select="list/string">
         <member><filename>
           <!-- Hyperlink the filename either to the NixOS Subversion