Python: Fix "unused disjunct" warnings

For the most part, these boil down to "some global property holds, and
so this relation contains all instances of class `X`". The fix is to
explicitly build the cartesian product (which we were already building
implicitly anyway) by adding `and exists(var)` to the disjunct that did
not mention `var`.

Note that these cartesian products are always with singletons on one
side, and so should be unproblematic.

python/ql/lib/semmle/python/Module.qll

@@ -196,7 +196,7 @@ private predicate isPotentialSourcePackage(Folder f) {
 private predicate isPotentialPackage(Folder f) {
   exists(f.getFile("__init__.py"))
   or
-  py_flags_versioned("options.respect_init", "False", _) and major_version() = 2
+  py_flags_versioned("options.respect_init", "False", _) and major_version() = 2 and exists(f)
 }
 
 private string moduleNameFromBase(Container file) {

python/ql/lib/semmle/python/dataflow/old/Legacy.qll

@@ -15,7 +15,7 @@ deprecated class CallContext extends TaintTrackingContext {
       )
     )
     or
-    this.isTop()
+    this.isTop() and exists(s)
   }
 }
 

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

@@ -124,7 +124,7 @@ int version_tuple_compare(Object t) {
 predicate baseless_is_new_style(ClassObject cls) {
   cls.isBuiltin()
   or
-  major_version() = 3
+  major_version() = 3 and exists(cls)
   or
   exists(cls.declaredMetaClass())
 }

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

@@ -329,7 +329,7 @@ private class ClassListList extends TClassListList {
       result = ConsList(removed_head, removed_tail)
     )
     or
-    this = EmptyList() and result = EmptyList()
+    this = EmptyList() and result = EmptyList() and exists(cls)
   }
 
   pragma[nomagic]

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

@@ -1076,7 +1076,7 @@ module InterProceduralPointsTo {
   /** Helper for default_parameter_points_to */
   pragma[noinline]
   private predicate context_for_default_value(ParameterDefinition def, PointsToContext context) {
-    context.isRuntime()
+    context.isRuntime() and exists(def)
     or
     exists(PointsToContext caller, CallNode call, PythonFunctionObjectInternal func, int n |
       context.fromCall(call, func, caller) and
@@ -2286,7 +2286,7 @@ module Types {
       func != six_add_metaclass_function() and result = false
     )
     or
-    not exists(Module m | m.getName() = "six") and result = false
+    not exists(Module m | m.getName() = "six") and result = false and exists(cls)
     or
     exists(Class pycls |
       pycls = cls.getScope() and