Internal importer creates an unusable backend when using xmi:id

[yvrng]

The import behaves differently depending on the type of file being read:

• With a standard XMI (using XPath), the import behaves correctly, and the resulting backend is the same as if we had used EMF.
• With an identified XMI (using xmi:id) the links between a container and a containment aren't done properly: When an original container expects several elements, the provided container has none.

[yvrng]

Partially solved in c01de65: References are first processed as attributes when reading, then redirected in EcoreProcessor which has access to its real type.

[yvrng]

Fixed in #69: the XMI reader no longer try to detect references, it simply delegate the structural features to the next processors