add support for bundle namespace alias

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

This is a backwards compatible change for adding support for namespace alias as with auto-mapped documents in the default Document folder.

The alias only work with https://github.com/symfony/symfony/pull/10853 , are ignored otherwise. Lets still wait for the symfony core PR to be merged before merging this.
I think `$alias` should be `$aliases` no?