Python: Fix all metrics-related compilation failures

In hindsight, having a `.getMetrics()` method that just returns `this`
is somewhat weird. It's possible that it predates the existence of the
inline cast, however.

python/ql/src/analysis/Summary.ql

@@ -3,6 +3,7 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from string key, string value
 where