Python: SSA adapter: add MultiAssignmentDefinition, definedBy, useOfDef

Extends the ESSA-shaped adapter on top of the new shared SSA with the
remaining APIs consumed by the dataflow library:

  * MultiAssignmentDefinition: matches the AST pattern 'a, b = ...' where
    the LHS is a Tuple/List and the Name being defined is a sub-element.
    Used by IterableUnpacking.qll to recognise unpacking assignments.

  * EssaNodeDefinition.definedBy(var, defNode): a flatter equivalent of
    'getSourceVariable() = var and getDefiningNode() = defNode', matching
    legacy ESSA's signature. Used by DataFlowPublic.qll's
    ModuleVariableNode to enumerate writes of a global.

  * AdjacentUses::useOfDef(def, use): all reachable uses of a definition
    (firstUse plus transitive use-use adjacency). Used by guards in
    DataFlowPublic.qll.

These complete the API surface enumerated by grep across the dataflow
library. The remaining items (EssaNodeRefinement, EssaImportStep) are
ImportResolution-specific and will need separate treatment, possibly via
a different abstraction since the SSA library does not model heap-state
refinements like 'foo.bar = X'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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

@@ -238,6 +238,15 @@ class EssaNodeDefinition extends Ssa::WriteDefinition {
   Py::Scope getScope() {
     exists(Cfg::ControlFlowNode n | n = this.getDefiningNode() | result = n.getScope())
   }
+
+  /**
+   * Holds if this definition defines source variable `v` at CFG node
+   * `defNode`. Flatter form of `getSourceVariable()` +
+   * `getDefiningNode()`, matching legacy ESSA's `definedBy`.
+   */
+  predicate definedBy(SsaSourceVariable v, Cfg::ControlFlowNode defNode) {
+    v = this.getSourceVariable() and defNode = this.getDefiningNode()
+  }
 }
 
 /**
@@ -301,6 +310,23 @@ class WithDefinition extends EssaNodeDefinition {
   }
 }
 
+/**
+ * An assignment where the LHS is a tuple/list and the RHS is unpacked:
+ * `a, b = (1, 2)` or `a, *rest = xs`. The defining node for each
+ * captured Name is the Name itself.
+ */
+class MultiAssignmentDefinition extends EssaNodeDefinition {
+  MultiAssignmentDefinition() {
+    exists(Cfg::NameNode n | n = this.getDefiningNode() |
+      exists(Py::Assign a, Py::Expr lhs |
+        a.getATarget() = lhs and
+        (lhs instanceof Py::Tuple or lhs instanceof Py::List) and
+        lhs.getASubExpression+() = n.getNode()
+      )
+    )
+  }
+}
+
 /**
  * An implicit entry definition for a non-local / captured / global /
  * builtin variable read in a scope but not defined there.
@@ -384,4 +410,14 @@ module AdjacentUses {
       use = bb.getNode(i)
     )
   }
+
+  /**
+   * Holds if `use` is any reachable use of definition `def`. Combines
+   * `firstUse` with transitive use-use adjacency.
+   */
+  predicate useOfDef(Ssa::Definition def, Cfg::NameNode use) {
+    firstUse(def, use)
+    or
+    exists(Cfg::NameNode mid | useOfDef(def, mid) and adjacentUseUse(mid, use))
+  }
 }