Moved migration helpers into their own namespace

Technically an API breakage, but I don't see migration helpers as part of our API surface.
They're one off utilitie

src/Dev/Tasks/MigrateFileTask.php

@@ -2,21 +2,15 @@
 
 namespace SilverStripe\Dev\Tasks;
 
-use Monolog\Formatter\FormatterInterface;
 use Monolog\Handler\StreamHandler;
 use Monolog\Logger;
 use Psr\Log\LoggerInterface;
-use Psr\Log\NullLogger;
 use SilverStripe\AssetAdmin\Helper\ImageThumbnailHelper;
-use SilverStripe\Assets\LegacyThumbnailMigrationHelper;
+use SilverStripe\Assets\Migration\LegacyThumbnailMigrationHelper;
+use SilverStripe\Assets\Migration\FileMigrationHelper;
 use SilverStripe\Assets\Storage\AssetStore;
 use SilverStripe\Control\Director;
-use SilverStripe\Core\Injector\Injector;
-use SilverStripe\Core\Tests\Injector\InjectorTest;
-use SilverStripe\Logging\HTTPOutputHandler;
 use SilverStripe\Logging\PreformattedEchoHandler;
-use SilverStripe\ORM\DB;
-use SilverStripe\Assets\FileMigrationHelper;
 use SilverStripe\Dev\BuildTask;
 
 /**