Implement basic control flow checks for consider-using-with

[DudeNr33]

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Type of Changes

	Type
✓	🐛 Bug fix

Description

The checker keeps track of context managers that have been assigned to a variable to see if they are later on used in a with block.
If a variable is reassigned before it is used in a with, the message will trigger (as intended).
However, the logic to check that was pretty basic, and did not handle cases where e.g. the variable is assigned inside two separate branches of control flow, like in if-else statements or try-except constructs.

This MR solves this. As the logic to determine if two nodes are in different branches of control flow might be useful in other places as well, I put the method inside the pylint.checkers.utils module.

Closes #4751

[coveralls]

Pull Request Test Coverage Report for Build 1179761563

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 92.733%

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Totals
Change from base Build 1177664240:	0.002%
Covered Lines:	13462
Relevant Lines:	14517

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💛 - Coveralls

[DudeNr33]

Oh wow, I just noticed that astroid already has a very similar method are_exclusive.
It covers not exactly the same cases (especially as it is focused on statements instead of notes, therefore it does not handle IfExp), but duplicating the logic inside pylint is of course nonsense.
I will refactor.

[Pierre-Sassoulas]

Thank you for recreating an astroid function then refactoring everything to use astroid :D The final result is pretty clean.