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/DataFlowPublic.qll

@@ -440,13 +440,17 @@ class ModuleVariableNode extends Node, TModuleVariableNode {
 
   /** Gets a node that reads this variable. */
   Node getARead() {
-    result.asCfgNode() = var.getALoad().getAFlowNode() and
-    // Ignore reads that happen when the module is imported. These are only executed once.
-    not result.getScope() = mod
+    result = this.getALocalRead()
     or
     this = import_star_read(result)
   }
 
+  /** Gets a node that reads this variable, excluding reads that happen through `from ... import *`. */
+  Node getALocalRead() {
+    result.asCfgNode() = var.getALoad().getAFlowNode() and
+    not result.getScope() = mod
+  }
+
   /** Gets an `EssaNode` that corresponds to an assignment of this global variable. */
   Node getAWrite() {
     any(EssaNodeDefinition def).definedBy(var, result.asCfgNode().(DefinitionNode))

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 = any(ModuleVariableNode v).getALocalRead()
     or
     // We include all scope entry definitions, as these act as the local source within the scope they
     // enter.