Fix detect bundles in configuration

As spotted by @wcluijt for the BlockBundle (https://github.com/symfony-cmf/BlockBundle/issues/121) the bundles are not detected correctly.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A