Speed up channel processing

In particular the /pkg action is now O(lg n) instead of O(n) in the
number of packages in the channel, and listing the channel contents
no longer requires calling isValidPath() on all packages.

Derivations (and thus build time dependencies) are no longer included
in the channel, because they're not GC roots.  Thus they could
disappear unexpectedly.

src/lib/Hydra/Base/Controller/ListBuilds.pm

@@ -76,12 +76,10 @@ sub nix : Chained('get_builds') PathPart('channel') CaptureArgs(1) {
     eval {
         if ($channelName eq "latest") {
             $c->stash->{channelName} = $c->stash->{channelBaseName} . "-latest";
-            getChannelData($c, scalar($c->stash->{latestSucceeded}));
+            $c->stash->{channelBuilds} = $c->stash->{latestSucceeded}
+                ->search_literal("exists (select 1 from buildproducts where build = me.id and type = 'nix-build')")
+                ->search({}, { columns => [@buildListColumns, 'drvpath', 'outpath', 'description', 'homepage'] });
         }
-        #elsif ($channelName eq "all") {
-        #    $c->stash->{channelName} = $c->stash->{channelBaseName} . "-all";
-        #    getChannelData($c, scalar($c->stash->{allBuilds}));
-        #}
         else {
             notFound($c, "Unknown channel `$channelName'.");
         }