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/libraries/Zope.qll

@@ -1,7 +1,7 @@
 /** Utilities for handling the zope libraries */
 
 import python
-private import semmle.python.pointsto.PointsTo
+private import LegacyPointsTo
 
 /** A method that belongs to a sub-class of `zope.interface.Interface` */
 class ZopeInterfaceMethodValue extends PythonFunctionValue {