Add new directory structure handler

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #117
| License       | MIT

This is a first move toward https://github.com/symfony/symfony-standard/issues/584
This allows the use of the new directory structure (not by default).

Since this changes the composer.json, everyone will automatically get a `Warning: The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. Run update to update them.` on the next install.