Protobuf:
  (package=Package)?
  (imports+=Import)*
  (options+=Option)*
  (elements+=ProtobufElement)*;
  
Package:
  'package' name=QualifiedName ';';

QualifiedName:
  ID ('.' ID)*;

ProtobufElement:
  Type | ExtendMessage;

Type:
  Message | Enum;

Message:
  'message' name=ID '{'
  elements+=MessageElement*
  ('extensions' extensionsFrom=INT 'to' (extensionsTo=INT | 'max') ';')?
  '}';

  @Override public QualifiedName getFullyQualifiedName(EObject obj) {
    QualifiedName fqn = super.getFullyQualifiedName(obj);
    if (fqn == null || obj instanceof Package) return fqn;
    Package p = finder.findPackage(obj);
    if (p == null) return fqn;
    List<String> segments = new ArrayList<String>(fqn.getSegments());
    segments.add(0, p.getName());
    return QualifiedName.create(segments.toArray(new String[segments.size()]));
  }

Message:
  'message' name=ID '{'
  otherMessage=[Message|QualifiedName]
  ('extensions' extensionsFrom=INT 'to' (extensionsTo=INT | 'max') ';')?
  '}';

public class MyDslRuntimeModule extends org.xtext.example.mydsl.AbstractMyDslRuntimeModule {

	@Override
	public Class<? extends IQualifiedNameProvider> bindIQualifiedNameProvider() {
		return MyQNP.class;
	}
	
}

package other;
message b
{
	test.a
	extensions 1 to 2;
}

package test;
message a
{
	other.b
	extensions 1 to 2;
}