Recursive source code inspection

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:
```python
$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():
...
```