Tag dependency injection classes as internal

[greg0ire]

They are not meant to be used directly by project code.

This is prompted by #480 (comment)

maybe more

src
├── Command
│   └── LoadDataFixturesDoctrineCommand.php
├── DependencyInjection
│   ├── CompilerPass
│   │   ├── FixturesCompilerPass.php
│   │   └── PurgerFactoryCompilerPass.php
│   └── DoctrineFixturesExtension.php
├── DoctrineFixturesBundle.php
├── FixtureGroupInterface.php
├── Fixture.php
├── Loader
│   └── SymfonyFixturesLoader.php
├── ORMFixtureInterface.php
└── Purger
    ├── ORMPurgerFactory.php
    └── PurgerFactory.php

Maybe the command and the bundle? Or maybe not… I'm not sure where to draw the line. I will also mention that yesterday, I saw this issue raised by somebody who uses a compiler pass in their own code.

I'm targeting 4.0.x, because even if adding @internal is maybe not really a breaking change, once a class is tagged as internal, it would suddenly make it OK to make it final.

[SenseException]

I see compiler passes mostly as internal by nature. Same with extension or configuration class. I'm not a fan of stunts where these get overwritten/repurposed in one or another way.