Improve external function detector: merge together shadowed function

[montyly]

For example

contract C{
    function f() public{

    }
}

contract D is C{
    function f() public{
        
    }
}

contract E{
    function f() public{
        
    }
}

Will result in

f() should be declared external:
	- C.f() (a.sol#2-4)
	- D.f() (a.sol#8-10)
f() should be declared external:
	- E.f() (a.sol#14-16)

The goal is to facilitate review of the finding. If the visibility is changed in the most derived contract, it must be changed in the base too. It will help the upcoming slither-format tool