Implement update mirrors/nothing/lock as its own installer mode

These special commands no longer (ab)use the partial update mechanism
but rather create a special install request for all current lock file
contents and later override any modified code references to the
originals. This leads to up to date remote metadata but no other
changes.

src/Composer/Command/UpdateCommand.php

@@ -121,6 +121,19 @@ EOT
             }
         }
 
+        // the arguments lock/nothing/mirrors are not package names but trigger a mirror update instead
+        // they are further mutually exclusive with listing actual package names
+        $filteredPackages = array_filter($packages, function ($package) {
+            return !in_array($package, array('lock', 'nothing', 'mirrors'), true);
+        });
+        $updateMirrors = $input->getOption('lock') || count($filteredPackages) != count($packages);
+        $packages = $filteredPackages;
+
+        if ($updateMirrors && !empty($packages)) {
+            $io->writeError('<error>You cannot simultaneously update only a selection of packages and regenerate the lock file metadata.</error>');
+            return -1;
+        }
+
         $commandEvent = new CommandEvent(PluginEvents::COMMAND, 'update', $input, $output);
         $composer->getEventDispatcher()->dispatch($commandEvent->getName(), $commandEvent);
 
@@ -146,7 +159,8 @@ EOT
             ->setClassMapAuthoritative($authoritative)
             ->setApcuAutoloader($apcu)
             ->setUpdate(true)
-            ->setUpdateWhitelist($input->getOption('lock') ? array('lock') : $packages)
+            ->setUpdateMirrors($updateMirrors)
+            ->setUpdateWhitelist($packages)
             ->setWhitelistTransitiveDependencies($input->getOption('with-dependencies'))
             ->setWhitelistAllDependencies($input->getOption('with-all-dependencies'))
             ->setIgnorePlatformRequirements($input->getOption('ignore-platform-reqs'))