Refactor/unify stub discovery #15691

Job	Run time
stubtest: stdlib (macos-latest, 3.12)	42s
stubtest: stdlib (macos-latest, 3.15)	1m 9s
stubtest: stdlib (macos-latest, 3.11)	34s
stubtest: stdlib (windows-latest, 3.13)	1m 9s
stubtest: stdlib (windows-latest, 3.11)	1m 9s
stubtest: stdlib (macos-latest, 3.13)	25s
stubtest: stdlib (windows-latest, 3.14)	1m 16s
stubtest: stdlib (windows-latest, 3.15)	1m 38s
stubtest: stdlib (windows-latest, 3.12)	1m 22s
stubtest: stdlib (macos-latest, 3.10)	40s
stubtest: stdlib (ubuntu-latest, 3.12)	38s
stubtest: stdlib (ubuntu-latest, 3.14)	38s
stubtest: stdlib (ubuntu-latest, 3.10)	40s
stubtest: stdlib (macos-latest, 3.14)	23s
stubtest: stdlib (ubuntu-latest, 3.15)	45s
stubtest: stdlib (ubuntu-latest, 3.11)	39s
stubtest: stdlib (ubuntu-latest, 3.13)	43s
	14m 30s

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