Allow user to set the project dir

Currently, the project directory is defined by the location of the composer.json file.
That file is not required in production, which therefore breaks the method getProjectDir (who sends back null).
This does not fix the behaviour, but allows the developer to pass the project dir as a parameter.

src/Symfony/Component/HttpKernel/Kernel.php

@@ -83,12 +83,13 @@ abstract class Kernel implements KernelInterface, RebootableInterface, Terminabl
     const END_OF_MAINTENANCE = '07/2019';
     const END_OF_LIFE = '01/2020';
 
-    public function __construct(string $environment, bool $debug)
+    public function __construct(string $environment, bool $debug, string $projectDir = null)
     {
         $this->environment = $environment;
         $this->debug = $debug;
         $this->rootDir = $this->getRootDir(false);
         $this->name = $this->getName(false);
+        $this->projectDir = $projectDir;
     }
 
     public function __clone()