Refactor/unify stub discovery #15687

Job	Run time
stubtest: stdlib (ubuntu-latest, 3.11)	41s
stubtest: stdlib (ubuntu-latest, 3.15)	43s
stubtest: stdlib (ubuntu-latest, 3.12)	34s
stubtest: stdlib (ubuntu-latest, 3.10)	42s
stubtest: stdlib (ubuntu-latest, 3.14)	35s
stubtest: stdlib (windows-latest, 3.13)	1m 47s
stubtest: stdlib (macos-latest, 3.12)	43s
stubtest: stdlib (macos-latest, 3.11)	36s
stubtest: stdlib (windows-latest, 3.11)	3m 4s
stubtest: stdlib (windows-latest, 3.14)	1m 29s
stubtest: stdlib (macos-latest, 3.15)	1m 8s
stubtest: stdlib (windows-latest, 3.12)	1m 7s
stubtest: stdlib (windows-latest, 3.15)	1m 42s
stubtest: stdlib (macos-latest, 3.10)	46s
stubtest: stdlib (macos-latest, 3.13)	46s
stubtest: stdlib (macos-latest, 3.14)	23s
stubtest: stdlib (ubuntu-latest, 3.13)	32s
	17m 18s

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