python: instantiate module for variable capture

This provides variable capture in standard situations:
- nested functions
- lambdas
There are some deficiencies:
- we do not yet handle objects capturing variables.
- we do not handle variables captured via the `nonlocal` keyword.
  This should be solved at the AST level, though, and then it
  should "just work".

There are still inconsistencies in the case where
a `SynthesizedCaptureNode` has a comprehensions
as its enclosing callable. In this case,
`TFunction(cn.getEnclosingCallable())` is not
defined and so getEnclosingCallable does not exist
for the `CaptureNode`.

python/ql/test/experimental/dataflow/variable-capture/in.py

@@ -34,17 +34,17 @@ def SINK_F(x):
 def inParam(tainted):
     def captureIn1():
         sinkI1 = tainted
-        SINK(sinkI1) #$ MISSING:captured
+        SINK(sinkI1) #$ captured
     captureIn1()
 
     def captureIn2():
         def m():
             sinkI2 = tainted
-            SINK(sinkI2) #$ MISSING:captured
+            SINK(sinkI2) #$ captured
         m()
     captureIn2()
 
-    captureIn3 = lambda arg: SINK(tainted)
+    captureIn3 = lambda arg: SINK(tainted) #$ captured
     captureIn3("")
 
     def captureIn1NotCalled():
@@ -68,17 +68,17 @@ def inLocal():
 
     def captureIn1():
         sinkI1 = tainted
-        SINK(sinkI1) #$ MISSING:captured
+        SINK(sinkI1) #$ captured
     captureIn1()
 
     def captureIn2():
         def m():
             sinkI2 = tainted
-            SINK(sinkI2) #$ MISSING:captured
+            SINK(sinkI2) #$ captured
         m()
     captureIn2()
 
-    captureIn3 = lambda arg: SINK(tainted) #$ MISSING:captured
+    captureIn3 = lambda arg: SINK(tainted) #$ captured
     captureIn3("")
 
     def captureIn1NotCalled():