makes doctrine less dependent upon the symfony yaml component

[thealjey]

This is the last place in the framework where the class is just too tightly coupled with the symfony yaml component, that the yaml parser alone cannot be overridden without overriding an entire class.
I simply brought it in accordance with "Doctrine\ORM\Mapping\Driver\YamlDriver" where a separate method called loadMappingFile exists to parse the mapping file.

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[Ocramius]

@thealjey what exactly is the use case for a custom YAML dumper here? Wouldn't it be simpler to just dump->import->re-dump with transformations?

[thealjey]

@Ocramius it certainly would not, because in that case:

1. the array would be dumped into a yaml string, just to be converted back to an array (which is what we originally needed), and then maybe after some modifications it would have to be dumped back into a string all over again.
2. the symfony yaml component needs to be available, you can't just swap it out with something else (maybe a different implementation of the same parser), which is exactly what I meant by "tightly coupled".

[Ocramius]

@thealjey seems legit