Python: Fix module level flow for iterable unpacking

(and for * patterns in match)

Since `PhaseDependentFlow` uses the following predicate, that relies on
.getScope() to be present for there to be any importTimeFlow (flow at
toplevel scope), it's important that data-flow nodes implement `.getScope`.

```
private predicate isTopLevel(Node node) { node.getScope() instanceof Module }
```

By implementing getScope, we can now rely on default implementation of
`getEnclosingCallable` in DataFlow::Node:

```
  /** Gets the enclosing callable of this node. */
  DataFlowCallable getEnclosingCallable() { result = getCallableScope(this.getScope()) }
```

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -467,7 +467,7 @@ class IterableSequenceNode extends Node, TIterableSequenceNode {
 
   override string toString() { result = "IterableSequence" }
 
-  override DataFlowCallable getEnclosingCallable() { result = consumer.getEnclosingCallable() }
+  override Scope getScope() { result = consumer.getScope() }
 
   override Location getLocation() { result = consumer.getLocation() }
 }
@@ -484,7 +484,7 @@ class IterableElementNode extends Node, TIterableElementNode {
 
   override string toString() { result = "IterableElement" }
 
-  override DataFlowCallable getEnclosingCallable() { result = consumer.getEnclosingCallable() }
+  override Scope getScope() { result = consumer.getScope() }
 
   override Location getLocation() { result = consumer.getLocation() }
 }
@@ -499,7 +499,7 @@ class StarPatternElementNode extends Node, TStarPatternElementNode {
 
   override string toString() { result = "StarPatternElement" }
 
-  override DataFlowCallable getEnclosingCallable() { result = consumer.getEnclosingCallable() }
+  override Scope getScope() { result = consumer.getScope() }
 
   override Location getLocation() { result = consumer.getLocation() }
 }

python/ql/test/experimental/dataflow/coverage/module_level.py

@@ -3,5 +3,5 @@
 
 t = (SOURCE, NONSOURCE)
 a, b = t
-SINK(a) #$ MISSING: flow="SOURCE, l:-2 -> a"
+SINK(a) #$ flow="SOURCE, l:-2 -> a"
 SINK_F(b)