Prepare LocalSourceNode for locality

Removes the dependence on the (global) `ModuleVariableNode.getARead()`.
In the charpred we simply inline it. In `localSourceFlowStep` we discard
any step _into_ a local source node.

Author: tausbn

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

@@ -67,7 +67,7 @@ class LocalSourceNode extends Node {
     or
     // We explicitly include any read of a global variable, as some of these may have local flow going
     // into them.
-    this = any(ModuleVariableNode mvn).getARead()
+    this.asCfgNode() = any(GlobalVariable v).getALoad().getAFlowNode()
     or
     // We include all scope entry definitions, as these act as the local source within the scope they
     // enter.