Allow user to set the project dir

[tdutrion]

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).
The offered solution, while working, requires the developer to implement it, and uses inheritance override, while a more aesthetic solution could be used.

This does not fix the behaviour, but allows the developer to pass the project dir as a parameter.

While this solution does not include BC break or anything, it is important to notice that it includes
an optional parameter.

Object instantiation in the framework bundle recipe could be updated as follow (in another PR):

$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG']);

$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG'], dirname(__DIR__));

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the master branch.