Add new directory structure hook

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #584
| License       | MIT

This requires https://github.com/sensiolabs/SensioDistributionBundle/pull/118 to be merged
Since the hook class gets removed, this will create an error when composer create-project is executed a second time (which doesn't hurt normally).