Refactoring in preparation for @Pure annotations

[lukarade]

Is your feature request related to a problem?

Right now, the visit_ast method handles both, traversing the ast-tree and handling its knots. This is inefficient and confusing in some parts.

Desired solution

The method visit_ast should be reworked: traversing and handling the ast-tree should be separated.
Therefor compare: https://github.com/lars-reimann/api-editor/blob/main/package-parser/package_parser/utils/_ASTWalker.py

• mark all relevant function (those who do not just handle traversing)
• examine all build in functions
• check all Astroid knots for impurity
• TBC.

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