[WIP][DependencyInjection] added a simple way to replace a service by keeping a reference to the old one

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5920
| License       | MIT
| Doc PR | not yet

This PR replaces #9003. Here's the todo list:
 - [ ] add a cookbook entry for this new feature
 - [ ] add support in all loaders/dumpers
 - [ ] add unit tests
 - [x] see if there are use cases in Symfony that would benefit from this new feature
 - [x] find the best name for this feature

I've implemented YAML and XML Loader / Dumper. 
From what I see, PhpDumper, PhpLoader, IniLoader, GraphvizDumper do not require an update, am I wrong?