pkg/manager: split off diff fuzzer functionality #19052

Job	Run time
race	10m 2s
aux	3m 55s
build	4m 9s
race_dashboard	3m 13s
dashboard	2m 20s
old	7m 6s
gvisor	4m 19s
arch (presubmit_arch_darwin)	4m 15s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_linux)	11m 12s
arch (presubmit_arch_freebsd)	6m 31s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_netbsd)	4m 2s
arch (presubmit_arch_executor)	3m 25s
	1h 11m 50s

Provide feedback