Python: Remove points-to references from python.qll

For now, these have just been made into `private` imports. After doing
this, I went through all of the (now not compiling) files and added in
private imports to the modules that they actually depended on.

I also added an explicit import of `LegacyPointsTo` (even though it may
be unnecessary) in cases where the points-to dependency was somewhat
surprising (and one we want to get rid of). This was primarily inside
the various SSA layers.

For modules inside `semmle.python.{types, objects, pointsto}` I did not
bother, as these are fairly clearly related to points-to.

python/ql/lib/semmle/python/SelfAttribute.qll

@@ -5,6 +5,7 @@
 
 import python
 private import semmle.python.pointsto.Filters
+private import LegacyPointsTo
 
 /**
  * An attribute access where the left hand side of the attribute expression