feature #21289 [DI] Add prototype services for PSR4-based discovery and registration (nicolas-grekas)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[DI] Add prototype services for PSR4-based discovery and registration

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | to be done

Talking with @dunglas, we wondered if this could be a good idea, as a more general approach to folder-based service registration as done in [DunglasActionBundle](https://github.com/dunglas/DunglasActionBundle/blob/master/DependencyInjection/DunglasActionExtension.php).

So here is the implementation.

This allows one to define a set of services as such:
```yaml
services:
    App\:
        resources: ../src/{Controller,Command} # relative to the current file as usual
        autowire: true # or any other attributes really
```

This looks for php files in the "src" folder, derivates PSR-4 class names from them, and uses `class_exists` for final discovery. The resulting services are named after the classes found this way.

The "resource" attribute can be a glob to select only a subset of the classes/files.

This approach has several advantages over [DunglasActionBundle](https://github.com/dunglas/DunglasActionBundle/blob/master/DependencyInjection/DunglasActionExtension.php):
- it is resilient to missing parent classes (see test case)
- it loads classes using the normal code path (the autoloader), thus play well with them (e.g. if one registered a special autoloader).
- it is more predictable, because it uses discovered paths as the only source of ids/classes to register - vs relying on `get_declared_classes`, which would make things context sensitive.

Fits well with current initiatives to me.

Commits
-------

03470b788e [DI] Add "psr4" service attribute for PSR4-based discovery and registration

Author: fabpot

Loader/FileLoader.php

@@ -32,7 +32,7 @@ abstract class FileLoader extends Loader
      */
     protected $locator;
 
-    private $currentDir;
+    protected $currentDir;
 
     /**
      * Constructor.