feature #21265 [DI] Implement PSR-11 (greg0ire)

This PR was merged into the 3.3-dev branch.

Discussion
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[DI] Implement PSR-11

TODO:

- [x] wait for a stable version of the psr/container package;
- [x] ~~deprecate instanciating ServiceNotFoundException directly, or using any of its methods directly;~~ not relevant anymore
- [x] act on the outcome of php-fig/container#8 (solved in php-fig/container#9) ;
- [x] ~~solve the mandatory NotFoundExceptionInterface on non-existing service
problem (with a flag ?);~~ non-issue, see comments below
- [x] provide meta-package psr/container-implementation if all problems can
be solved.

| Q             | A
| ------------- | ---
| Branch?       | master
| New feature?  | yes
| BC breaks?    | not at the moment
| Tests pass?   | didn't pass before pushing, or even starting to code, will see Travis
| License       | MIT
| Doc PR        | TODO

This PR is a first attempt at implementing PSR-11, or at least trying to get closer to it.
Delegate lookup is optional, and thus not implemented for now.

Commits
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bde0efd01c Implement PSR-11

Author: fabpot

Resources/config/services.xml

@@ -40,6 +40,7 @@
             <argument type="collection" />
         </service>
 
+        <service id="Psr\Container\ContainerInterface" alias="service_container" public="false" />
         <service id="Symfony\Component\DependencyInjection\ContainerInterface" alias="service_container" public="false" />
         <service id="Symfony\Component\DependencyInjection\Container" alias="service_container" public="false" />