Refactor/unify stub discovery #15673

Job	Run time
stubtest: stdlib (windows-latest, 3.11)	1m 53s
stubtest: stdlib (ubuntu-latest, 3.10)	55s
stubtest: stdlib (ubuntu-latest, 3.13)	42s
stubtest: stdlib (ubuntu-latest, 3.11)	34s
stubtest: stdlib (macos-latest, 3.10)	48s
stubtest: stdlib (ubuntu-latest, 3.12)	34s
stubtest: stdlib (ubuntu-latest, 3.14)	45s
stubtest: stdlib (windows-latest, 3.15)	1m 42s
stubtest: stdlib (macos-latest, 3.13)	22s
stubtest: stdlib (ubuntu-latest, 3.15)	47s
stubtest: stdlib (macos-latest, 3.15)	48s
stubtest: stdlib (macos-latest, 3.12)	30s
stubtest: stdlib (windows-latest, 3.12)	1m 19s
stubtest: stdlib (macos-latest, 3.11)	40s
stubtest: stdlib (macos-latest, 3.14)	29s
stubtest: stdlib (windows-latest, 3.13)	2m 15s
stubtest: stdlib (windows-latest, 3.14)	1m 8s
	16m 11s

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