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

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no yet
| Fixed tickets | #6484, #3079, #9238, #10976, #10943, #10999, symfony/symfony-standard#659
| 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.

I still did not update the tests nor did not add new ones.
Before that, what do you think of this approach?