Refactor/unify stub discovery #15690

Job	Run time
stubtest: stdlib (windows-latest, 3.12)	1m 21s
stubtest: stdlib (windows-latest, 3.15)	1m 51s
stubtest: stdlib (ubuntu-latest, 3.13)	35s
stubtest: stdlib (windows-latest, 3.14)	1m 20s
stubtest: stdlib (windows-latest, 3.11)	1m 17s
stubtest: stdlib (ubuntu-latest, 3.12)	42s
stubtest: stdlib (ubuntu-latest, 3.11)	43s
stubtest: stdlib (ubuntu-latest, 3.14)	35s
stubtest: stdlib (ubuntu-latest, 3.15)	41s
stubtest: stdlib (macos-latest, 3.10)	1m 1s
stubtest: stdlib (macos-latest, 3.12)	54s
stubtest: stdlib (ubuntu-latest, 3.10)	39s
stubtest: stdlib (macos-latest, 3.13)	26s
stubtest: stdlib (macos-latest, 3.11)	35s
stubtest: stdlib (macos-latest, 3.15)	51s
stubtest: stdlib (macos-latest, 3.14)	23s
stubtest: stdlib (windows-latest, 3.13)	1m 4s
	14m 58s

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