Recursive source code inspection

[wajda]

The source code inspector should run recursively and inspect all called functions, including ones from other modules.
The result of the inspection should be a DAG where node represent functions (with their source code) and edges represent invocations.
The resulted DAG should be serialised as a plain pseudo python file in the format, and included into the source_code property of the python_function operation:

$The_source_code_of_the_root_function_fn_here

### ==============================================
### module: AAA
### ==============================================

from BBB import xxx


### func: f1
def f1():
    ...
    f2()
    ...


### func: f2 
def f2():
    ...
    xxx()
    ...


### ==============================================
### module: BBB
### ==============================================

### func: xxx
def xxx():
    ...