chore(import): make root discovery future proof

Related to https://github.com/bpmn-io/bpmn-js/issues/1546

lib/import/Importer.js

@@ -77,10 +77,13 @@ export function importBpmnDiagram(diagram, definitions, bpmnDiagram) {
 
     var mainDiagram = bpmnDiagram || definitions.diagrams[0];
 
+    var rootId = mainDiagram.plane.bpmnElement.id;
+
+    // we do need to account for different ways we create root elements
+    // each nested imported <root> do have the `_plane` suffix, while
+    // the root <root> is found under the business object ID
     canvas.setRootElement(
-      canvas.findRoot(
-        mainDiagram.plane.bpmnElement.id
-      )
+      canvas.findRoot(rootId + '_plane') || canvas.findRoot(rootId)
     );
   }