Python: Fix global flow

A slightly odd fix, but still morally okay, I think. The main issue
here was that global variables have their first occurrence in an inner
scope inside a so-called "scope entry definition", that then
subsequently flows to the first use of this variable. This meant that
that first use was _not_ a `LocalSourceNode` (since _something_ flowed
into it), and this blocked `trackUseNode` from type-tracking to it (as
it expects all nodes to be `LocalSourceNode`s).

The answer, then, is to say that a `LocalSourceNode` is simply one
that doesn't have flow to it from _any `CfgNode`_ (through one or more
steps). This disregards the flow from the scope entry definition, as
that is flow from an `EssaNode`.

Additionally, it makes sense to exclude `ModuleVariableNode`s. These
should never be considered local sources, since they always have flow
from (at least) the place where the corresponding global variable is
introduced.

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

@@ -443,7 +443,10 @@ class BarrierGuard extends GuardNode {
  * - Function parameters
  */
 class LocalSourceNode extends Node {
-  LocalSourceNode() { not simpleLocalFlowStep(_, this) }
+  LocalSourceNode() {
+    not simpleLocalFlowStep+(any(CfgNode n), this) and
+    not this instanceof ModuleVariableNode
+  }
 
   /** Holds if this `LocalSourceNode` can flow to `nodeTo` in one or more local flow steps. */
   cached