Python: Make some more points-to imports private

This makes things a bit cleaner.

After this, the only non-private (and non-`LegacyPointsTo`) imports of
`semmle.python.{types,objects,pointsto}.*` are in
`semmle.python.objects.ObjectInternal`, which is reasonable, as that is
the entry point for the entire internal object API.

Author: tausbn

python/ql/lib/semmle/python/pointsto/PointsTo.qll

@@ -6,6 +6,7 @@ private import semmle.python.pointsto.PointsToContext
 private import semmle.python.pointsto.MRO
 private import semmle.python.types.Builtins
 private import semmle.python.types.Extensions
+private import semmle.python.pointsto.Context
 private import semmle.python.internal.CachedStages
 private import semmle.python.types.Object
 private import semmle.python.types.FunctionObject

python/ql/lib/semmle/python/types/Extensions.qll

@@ -15,8 +15,8 @@ private import semmle.python.pointsto.PointsTo
 private import semmle.python.pointsto.PointsToContext
 private import semmle.python.objects.TObject
 /* Make ObjectInternal visible to save extra imports in user code */
-import semmle.python.objects.ObjectInternal
-import semmle.python.pointsto.Context
+private import semmle.python.objects.ObjectInternal
+private import semmle.python.pointsto.Context
 
 abstract class PointsToExtension extends @py_flow_node {
   string toString() { result = "PointsToExtension with missing toString" }