JSON-LD: Support top-level @graph expressing implicit default graph

Per W3C JSON-LD v1.1 §4.9 (non-normative ref):

    When a JSON-LD document's top-level structure is a map that contains
    no other keys than @graph and optionally @context (properties that
    are not mapped to an IRI or a keyword are ignored), @graph is
    considered to express the otherwise implicit default graph.

Support such a structure in InfoExtractor._json_ld parsing:
Wrap the control flow block in a function, which is called recursively
upon such a structure

youtube_dl/extractor/common.py

@@ -1296,8 +1296,16 @@ class InfoExtractor(object):
             })
             extract_interaction_statistic(e)
 
-        for e in json_ld:
+        def traverse_json_ld(json_ld, info, at_top_level=True):
-            if '@context' in e:
+            for e in json_ld:
+                if at_top_level and '@context' not in e:
+                    continue
+                if at_top_level and all(k in ('@context', '@graph') for k in e):
+                    graph = e['@graph']
+                    if isinstance(graph, dict):
+                        graph = [graph]
+                    traverse_json_ld(graph, info, at_top_level=False)
+                    break
                 item_type = e.get('@type')
                 if expected_type is not None and expected_type != item_type:
                     continue
@@ -1345,6 +1353,8 @@ class InfoExtractor(object):
                     continue
                 else:
                     break
+        traverse_json_ld(json_ld, info)
+
         return dict((k, v) for k, v in info.items() if v is not None)
 
     @staticmethod