Change simple map to a ChainMap for global_registry management
Change simple map to a ChainMap for global_registry management

When a ResourceSet is used, the search for metamodel uri follows this
schema:

* first the EPackage is searched in the rset local metamodel registry
* if the EPackage is not found, it switches to the the global_registry

This modification simply refactor code by using a ChainMap that proposes
this kind of management.

pyecore/pyecore,aranega/pyecore