Refactor/unify stub discovery #15689

Job	Run time
stubtest: stdlib (ubuntu-latest, 3.11)	41s
stubtest: stdlib (ubuntu-latest, 3.10)	42s
stubtest: stdlib (ubuntu-latest, 3.12)	41s
stubtest: stdlib (windows-latest, 3.11)	1m 49s
stubtest: stdlib (windows-latest, 3.15)	1m 46s
stubtest: stdlib (ubuntu-latest, 3.15)	50s
stubtest: stdlib (windows-latest, 3.13)	1m 28s
stubtest: stdlib (macos-latest, 3.14)	41s
stubtest: stdlib (macos-latest, 3.10)	45s
stubtest: stdlib (windows-latest, 3.12)	1m 25s
stubtest: stdlib (ubuntu-latest, 3.13)	38s
stubtest: stdlib (ubuntu-latest, 3.14)	34s
stubtest: stdlib (macos-latest, 3.15)	48s
stubtest: stdlib (windows-latest, 3.14)	1m 32s
stubtest: stdlib (macos-latest, 3.11)	42s
stubtest: stdlib (macos-latest, 3.13)	37s
stubtest: stdlib (macos-latest, 3.12)	36s
	16m 15s

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