Python: Remove bad join in getCallableScope

Clause timing report had this suspicious entry

```
CommandInjection.ql-12:DataFlowPublic::Node::getCallableScope#bbf .................. 7.2s
    (4 evaluations with max 6.4s in DataFlowPublic::Node::getCallableScope#bbf/3@i3#119d7b)
```

which indeed was a bad join:
```
Tuple counts for DataFlowPublic::Node::getCallableScope#bbf:
293509   ~2%     {3} r1 = JOIN DataFlowPublic::Node::getCallableScope#bbf#prev_delta AS L WITH DataFlowPublic::TNode#f AS R ON FIRST 1 OUTPUT L.<1>, L.<0>, L.<2>
22337162 ~0%     {3} r2 = JOIN r1 WITH Scope::Scope::getEnclosingScope_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT r1.<1>, r1.<2>, R.<1>
22337162 ~0%     {3} r3 = r2 AND NOT DataFlowPublic::Node::getCallableScope#bbf#prev AS R(r2.<0>, r2.<2>, r2.<1>)
22337162 ~0%     {3} r4 = SCAN r3 OUTPUT r3.<0>, r3.<2>, r3.<1>
722      ~1%     {3} r5 = JOIN r4 WITH m#DataFlowPublic::Node::getCallableScope#bbf AS R ON FIRST 2 OUTPUT r4.<0>, r4.<1>, r4.<2>
722      ~1%     {3} r6 = JOIN r5 WITH m#DataFlowPublic::Node::getCallableScope#bbf AS R ON FIRST 2 OUTPUT r5.<0>, r5.<2>, r5.<1>
722      ~1%     {3} r7 = r6 AND NOT project#DataFlowPrivate::DataFlowCallable::getScope_dispred#ff AS R(r6.<2>)
722      ~1%     {3} r8 = SCAN r7 OUTPUT r7.<0>, r7.<2>, r7.<1>
                 return r8
```

In this case, the join went away by simply moving the helper predicate
out of the class it was situated in (and since it doesn't mention
`this`, it didn't really belong there in the first place).

Result:
```
DataFlowPublic.qll-8:DataFlowPublic::getCallableScope#ff ........................... 26ms
    (4 evaluations with max 15ms in DataFlowPublic::getCallableScope#ff/2@i3#709a9e)
```

Author: tausbn

python/ql/src/experimental/dataflow/internal/DataFlowPublic.qll

@@ -62,6 +62,14 @@ newtype TNode =
     call_unpacks(call, _, callable, name, _)
   }
 
+/** Helper for `Node::getEnclosingCallable`. */
+private DataFlowCallable getCallableScope(Scope s) {
+  result.getScope() = s
+  or
+  not exists(DataFlowCallable c | c.getScope() = s) and
+  result = getCallableScope(s.getEnclosingScope())
+}
+
 /**
  * An element, viewed as a node in a data flow graph. Either an SSA variable
  * (`EssaNode`) or a control flow node (`CfgNode`).
@@ -73,13 +81,6 @@ class Node extends TNode {
   /** Gets the scope of this node. */
   Scope getScope() { none() }
 
-  private DataFlowCallable getCallableScope(Scope s) {
-    result.getScope() = s
-    or
-    not exists(DataFlowCallable c | c.getScope() = s) and
-    result = getCallableScope(s.getEnclosingScope())
-  }
-
   /** Gets the enclosing callable of this node. */
   DataFlowCallable getEnclosingCallable() { result = getCallableScope(this.getScope()) }