Add 'drush' directory in selected Drupal root and selected Composer r…

…oot to the alias manager's search path.

src/Preflight/Preflight.php

@@ -159,7 +159,9 @@ protected function doRun($argv)
         $aliasManager = (new SiteAliasManager())
             ->addSearchLocation($preflightArgs->aliasPath())
             ->addSearchLocation($this->environment->systemConfigPath())
-            ->addSearchLocation($this->environment->userConfigPath());
+            ->addSearchLocation($this->environment->userConfigPath())
+            ->addSearchLocation($this->selectedDrupalRoot() . '/drush')
+            ->addSearchLocation($this->selectedComposerRoot() . '/drush');
         $selfAliasRecord = $aliasManager->findSelf($preflightArgs->alias(), $root, $preflightArgs->uri());
         $aliasConfig = $selfAliasRecord->exportConfig();
         $configLocator->addAliasConfig($aliasConfig);
@@ -226,9 +228,19 @@ protected function findSelectedSite(PreflightArgs $preflightArgs)
     protected function setSelectedSite($selectedRoot)
     {
         $this->drupalFinder->locateRoot($selectedRoot);
+        return $this->selectedDrupalRoot();
+    }
+
+    protected function selectedDrupalRoot()
+    {
         return $this->drupalFinder->getDrupalRoot();
     }
 
+    protected function selectedComposerRoot()
+    {
+        return $this->drupalFinder->getComposerRoot();
+    }
+
     /**
      * Create a command file discovery object
      */