Python: Add overlay annotations to AST classes

... and everything else that it depends on.

python/ql/lib/semmle/python/essa/Definitions.qll

@@ -1,3 +1,6 @@
+overlay[local]
+module;
+
 import python
 /*
  * Classification of variables. These should be non-overlapping and complete.

python/ql/lib/semmle/python/essa/Essa.qll

@@ -1,6 +1,8 @@
 /**
  * Library for SSA representation (Static Single Assignment form).
  */
+overlay[local]
+module;
 
 import python
 private import SsaCompute

python/ql/lib/semmle/python/essa/SsaCompute.qll

@@ -88,6 +88,8 @@
  * ```
  * and thus it falls out that `g3` must be `1`.
  */
+overlay[local]
+module;
 
 import python
 private import semmle.python.internal.CachedStages

python/ql/lib/semmle/python/essa/SsaDefinitions.qll

@@ -2,6 +2,8 @@
  * Provides classes and predicates for determining the uses and definitions of
  * variables for ESSA form.
  */
+overlay[local]
+module;
 
 import python
 private import semmle.python.internal.CachedStages