pkg/manager: split off diff fuzzer functionality #19053

Job	Run time
build	3m 9s
dashboard	2m 11s
race	7m 23s
aux	3m 56s
gvisor	4m 12s
race_dashboard	4m 8s
old	7m 2s
arch (presubmit_arch_netbsd)	3m 43s
arch (presubmit_arch_openbsd)	3m 52s
arch (presubmit_arch_linux)	10m 24s
arch (presubmit_arch_freebsd)	6m 6s
arch (presubmit_arch_darwin)	3m 57s
arch (presubmit_arch_windows)	3m 40s
arch (presubmit_arch_executor)	3m 38s
	1h 7m 21s

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