Moves the classes/predicates that _actually_ depend on points-to to the
`LegacyPointsTo` module, leaving behind a module that contains all of
the metrics-related stuff (line counts, nesting depth, etc.) that don't
need points-to to be evaluated.
Consequently, `Metrics` is now no longer a private import in
`python.qll`.
Gets rid of a bunch of predicates relating to reachability (which
depended on the modelling of exceptions, which uses points-to), moving
them to `LegacyPointsTo`. In the process, we gained a new class
`BasicBlockWithPointsTo`.
Moves the existing points-to predicates to the newly added class
`ControlFlowNodeWithPointsTo` which resides in the `LegacyPointsTo`
module.
(Existing code that uses these predicates should import this module, and
references to `ControlFlowNode` should be changed to
`ControlFlowNodeWithPointsTo`.)
Also updates all existing points-to based code to do just this.
