Annotation imports are not parsed for nested traits

### Bug Report

When an entity uses a trait that itself also uses a trait with a doctrine annotation the mapper failes to recognize the imported annotation. Code example below if it's not clear.

| Q | A
|------------ | ------
| BC Break | no
| Version | 1.6.1

#### How to reproduce

Create the following class and traits using doctrine/orm:

```php
// ClassA.php

use Doctrine\Orm\Mapping\Entity;

/**
* @Entity
*/
class ClassA
{
use TraitA;
}
```

```php
// TraitA.php

trait TraitA
{
use TraitB;
}
```

```php
// TraitB.php

use Doctrine\ORM\Mapping\Column;

trait TraitB
{
/**
* @Column(name="column", type="string")
*/
protected $column;
}
```

Trying to create a mapping for this setup will throw an `AnnotationException`:
```
[Semantical Error] The annotation "@Column" in property Search\Model\ClassA::$column was never imported. Did you maybe forget to add a "use" statement for this annotation?
```

This will fail on any Doctrine annotation that is used in `TraitB` but not imported in `TraitA` or `ClassA`.

I expect the `@Column`-annotation imported in `TraitB` to stay also be valid for `ClassA` where it ends up in.