Merge logic of `add_cells` and `required_imports`

[lukany]

Logic of add_cells() and required_imports() is tightly coupled. E.g. see 9e8f50f where self.subsections_to_show_with_low_verbosity is used within both the methods. This kind of coupling introduces risk of bugs that the methods are not in sync and some imports are omitted (such as the linked PR is fixing).

I propose to merge the logic of the two methods. I am not sure how though. Maybe add_cells() can add both cells as well as imports that are required for the cells to execute successfully. required_imports() would then collect theses imports (from e.g. self._cell_imports()and potentially add some more (e.g. fromsuper`).