Hide "Packages" field in Diffusion Commits query form when application uninstalled

Summary: Fixes T15698

Test Plan:
1. Uninstall Packages via `/applications/view/PhabricatorPackagesApplication/`
2. Go to `/diffusion/commit/`, expand the Query form, and see that `Packages` is not shown anymore

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15698

Differential Revision: https://we.phorge.it/D25506

src/applications/audit/query/PhabricatorCommitSearchEngine.php

@@ -70,6 +70,8 @@ final class PhabricatorCommitSearchEngine
   }
 
   protected function buildCustomSearchFields() {
+    $show_packages = PhabricatorApplication::isClassInstalled(
+      'PhabricatorPackagesApplication');
     return array(
       id(new PhabricatorSearchDatasourceField())
         ->setLabel(pht('Responsible Users'))
@@ -118,6 +120,7 @@ final class PhabricatorCommitSearchEngine
         ->setConduitKey('packages')
         ->setAliases(array('package', 'packages', 'packagePHID'))
         ->setDatasource(new PhabricatorOwnersPackageDatasource())
+        ->setIsHidden(!$show_packages)
         ->setDescription(
           pht('Find commits which affect given packages.')),
       id(new PhabricatorSearchThreeStateField())