pkg/manager: refactor diff.go #19051

Job	Run time
race	7m 22s
gvisor	4m 1s
build	3m 19s
race_dashboard	3m 15s
aux	3m 39s
old	7m 15s
dashboard	2m 41s
arch (presubmit_arch_freebsd)	6m 32s
arch (presubmit_arch_netbsd)	4m 5s
arch (presubmit_arch_darwin)	3m 52s
arch (presubmit_arch_linux)	10m 46s
arch (presubmit_arch_windows)	3m 46s
arch (presubmit_arch_openbsd)	3m 48s
arch (presubmit_arch_executor)	3m 27s
	1h 7m 48s

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