FileDigestCheck: some more harmonization and type annotation #2455

Job	Run time
CI (normal, registry.opensuse.org/opensuse/tumbleweed:latest)	1m 37s
CI (registry.fedoraproject.org/fedora:latest, no-optional-deps)	1m 17s
CI (registry.opensuse.org/opensuse/tumbleweed:latest, no-optional-deps)	1m 9s
CI (normal, registry.fedoraproject.org/fedora:latest)	1m 42s
CI (normal, registry.fedoraproject.org/fedora:rawhide)	1m 48s
	7m 33s

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