[DependencyInjection] make paths relative to __DIR__ in the generated container

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6484, #3079, partially #9238, #10894, #10999
| License       | MIT
| Doc PR        | n/a

This is an alternative approach to #10999 for removing absolute paths from the generated container:
instead of trying to fix the container file after it has been dumped, telling to the PhpDumper where its output will be written allows it to replace parts of strings by an equivalent value based on `__DIR__`.
This should be safe, thus the PR is on 2.3.
Another minor comment: don't you think that a new line above this could improve code legibility?