Extract parallel execution into a dedicated class (#86)

A design issue with the current `Parallelization` trait is that:

- it relies on the Container for which the behaviour changed
- it provides little way to by-pass the Container which is a problem in a non Symfony App context
- it provides private code but it is actually not protected due to being in a trait
- a lot of verbose code has been added to accommodate the diversity of setups, which is reflected in the number of parameters passed
- the piece of code is a nightmare to test
- in the future we also want to provide a command instead of the trait. The trait can stay as it is handy and extending commands is sometimes awkward/impossible, but the UX could be made better via a command. However the current trait implementation does not play well with it.

I'm proposing to extract it into a dedicated class. The current implementation simply ported all the code as-is. I do plan to rework it to make easier to use.

Author: theofidry

phpstan.neon.dist

@@ -17,7 +17,8 @@ parameters:
         - path: tests/Integration/ParallelizationIntegrationTest.php
           message: '#ParallelizationIntegrationTest::\$noSubProcessCommand#'
 
+        - path: src/ParallelExecutor.php
+          message: '#Call to method reset\(\) on an unknown class .*ResettableContainerInterface#'
+
         - path: tests/ChunkedItemsIteratorTest.php
           message: '#Parameter \#2 \$fetchItems of static method .+ChunkedItemsIterator::fromItemOrCallable\(\) expects callable\(\)#'
-
-        - '#unknown class Symfony\\Component\\DependencyInjection\\ResettableContainerInterface#'

src/Logger/LoggerFactory.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * This file is part of the Webmozarts Console Parallelization package.
+ *
+ * (c) Webmozarts GmbH <office@webmozarts.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Webmozarts\Console\Parallelization\Logger;
+
+use Symfony\Component\Console\Output\OutputInterface;
+
+interface LoggerFactory
+{
+    public function create(OutputInterface $output): Logger;
+}