Python: Remove points-to dependency from parts of SSA

For whatever reason, the CFG node for exceptions and exception groups
was placed with the points-to code. (Probably because a lot of the
predicates depended on points-to.)

However, as it turned out, two of the SSA modules only depended on
non-points-to properties of these nodes, and so it was fairly
straightforward to remove the imports of `LegacyPointsTo` for those
modules.

In the process, I moved the aforementioned CFG node types into
`Flow.qll`, and changed the classes in the `Exceptions` module to the
`...WithPointsTo` form that we introduced elsewhere.

python/ql/test/2/library-tests/ControlFlow/Exceptions/Handles.ql

@@ -1,6 +1,6 @@
 import python
 private import LegacyPointsTo
 
-from ExceptFlowNode ex, Value val
+from ExceptFlowNodeWithPointsTo ex, Value val
 where ex.handledException(val, _, _)
 select ex.getLocation().getStartLine(), ex.toString(), val.toString()