Do not warn of mirrors not existing

src/Composer/Installer.php

@@ -1316,7 +1316,7 @@ class Installer
                 }
             }
 
-            if (count($depPackages) == 0 && !$nameMatchesRequiredPackage && !in_array($packageName, array('nothing', 'lock'))) {
+            if (count($depPackages) == 0 && !$nameMatchesRequiredPackage && !in_array($packageName, array('nothing', 'lock', 'mirrors'))) {
                 $this->io->writeError('<warning>Package "' . $packageName . '" listed for update is not installed. Ignoring.</warning>');
             }