Refactor writer._sweep_indirect_references: _idnum_hash and extern_map #1036
Description
In #207 we received valuable feedback that we might have code duplication: #207 (review)
It was not clarified within that PR as the PR improved PyPDF2 and I wanted to avoid a further delay (after 7 years of waiting!) with merging it.
This issue is only here as a reminder to have a look at it.