Try harder to find build logs

Due to the fixed-output derivation hashing scheme, there can be
multiple derivations of the same output path.  But build logs are
indexed by derivation path.  Thus, we may not be able to find the
log of a build or build step using its derivation.  So as a fallback,
Hydra now looks for other derivations with the same output paths.

src/root/build.tt

@@ -23,7 +23,7 @@
     <tbody>
       [% FOREACH step IN build.buildsteps %]
         [% IF ( type == "All" ) || ( type == "Failed" && step.status != 0 ) || ( type == "Running" && step.busy == 1 ) %]
-          [% has_log = log_exists(step.drvpath);
+          [% has_log = buildStepLogExists(step);
              log = c.uri_for('/build' build.id 'nixlog' step.stepnr); %]
           <tr>
             <td>[% step.stepnr %]</td>
@@ -177,7 +177,7 @@
                      finished at [% INCLUDE renderDateTime timestamp = actualBuild.stoptime %]</td>
             </tr>
             [% END %]
-            [% IF !isAggregate && log_exists(build.drvpath) %]
+            [% IF !isAggregate && buildLogExists(build) %]
               <tr>
                 <th>Logfile:</th>
                 <td>

src/root/deps.tt

@@ -12,7 +12,7 @@
       <span id="[% done.${node.path} %]"><span class="dep-tree-line">
         [% IF node.buildStep %]
           <a href="[% c.uri_for('/build' node.buildStep.get_column('build')) %]"><tt>[% node.name %]</tt></a> [%
-            IF log_exists(node.buildStep.drvpath);
+            IF buildStepLogExists(node.buildStep);
               INCLUDE renderLogLinks url=c.uri_for('/build' node.buildStep.get_column('build') 'nixlog' node.buildStep.stepnr);
             END %]
         [% ELSE %]