Code checking whether an ID should be considered as part of the
currently processed liboverride hierarchy or not was very similar all
over the liboverride code.
It is now deduplicated into two util functions, which helps ensuring
coherence in these checks, and future potential changes in this
filtering process.
NOTE: While no pratical changes are expected form user PoV with this
refactor, technically it does modifies the behavior in some cases (added
checks).
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