[HttpKernel] removed absolute paths from the generated container

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | first step to resolve #6484, #3079, and #9238
| License       | MIT
| Doc PR        | n/a

This PR converts absolute paths to relative ones in the dumped container. The code is a bit "ugly", but it gets the job done and I'm not sure that there is a more elegant way without breaking everything.