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/global.py

@@ -78,7 +78,7 @@ def through(tainted):
         global sinkT1
         sinkT1 = tainted
     captureOut1()
-    SINK(sinkT1) #$ MISSING:captured
+    SINK(sinkT1) #$ captured
 
     def captureOut2():
         def m():
@@ -86,7 +86,7 @@ def through(tainted):
             sinkT2 = tainted
         m()
     captureOut2()
-    SINK(sinkT2) #$ MISSING:captured
+    SINK(sinkT2) #$ captured
 
     def captureOut1NotCalled():
         global nonSinkT1