Python: Move Boolean into TypeTrackerPrivate

In general, this may be defined already for other languages, so moving
it in here will avoid potential clashes.

python/ql/src/experimental/typetracking/TypeTracker.qll

@@ -78,11 +78,6 @@ module StepSummary {
   }
 }
 
-/**
- * A utility class that is equivalent to `boolean` but does not require type joining.
- */
-private class Boolean extends boolean {
-  Boolean() { this = true or this = false }
 }
 
 private newtype TTypeTracker = MkTypeTracker(Boolean hasCall, OptionalContentName content)

python/ql/src/experimental/typetracking/TypeTrackerPrivate.qll

@@ -95,3 +95,10 @@ predicate basicLoadStep(Node nodeFrom, Node nodeTo, string content) {
     nodeTo = a
   )
 }
+
+/**
+ * A utility class that is equivalent to `boolean` but does not require type joining.
+ */
+class Boolean extends boolean {
+  Boolean() { this = true or this = false }
+}