Refactor/unify stub discovery #15674

Job	Run time
stubtest: stdlib (ubuntu-latest, 3.12)	43s
stubtest: stdlib (windows-latest, 3.11)	1m 21s
stubtest: stdlib (macos-latest, 3.13)	28s
stubtest: stdlib (windows-latest, 3.14)	1m 27s
stubtest: stdlib (ubuntu-latest, 3.10)	45s
stubtest: stdlib (windows-latest, 3.12)	1m 19s
stubtest: stdlib (ubuntu-latest, 3.13)	34s
stubtest: stdlib (windows-latest, 3.13)	1m 10s
stubtest: stdlib (macos-latest, 3.10)	49s
stubtest: stdlib (macos-latest, 3.14)	40s
stubtest: stdlib (ubuntu-latest, 3.14)	36s
stubtest: stdlib (ubuntu-latest, 3.15)	51s
stubtest: stdlib (macos-latest, 3.11)	34s
stubtest: stdlib (windows-latest, 3.15)	1m 51s
stubtest: stdlib (ubuntu-latest, 3.11)	37s
stubtest: stdlib (macos-latest, 3.12)	42s
stubtest: stdlib (macos-latest, 3.15)	46s
	15m 13s

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