[DependencyInjection] Perf php dumper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR came up after this comment to reduce the number of calls to dirname():
https://github.com/symfony/symfony/pull/12784#issuecomment-65619179
why at least 2 root dirs ?