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/Metrics/DocStringRatio.ql

@@ -11,9 +11,10 @@
  */
 
 import python
+private import LegacyPointsTo
 
-from Module m, ModuleMetrics mm
-where mm = m.getMetrics() and mm.getNumberOfLines() > 0
-select m,
+from ModuleMetrics mm
+where mm.getNumberOfLines() > 0
+select mm,
   100.0 * (mm.getNumberOfLinesOfDocStrings().(float) / mm.getNumberOfLines().(float)) as ratio
   order by ratio desc