Break up getChangedPackages internals #1984

Job	Run time
build docs	25s
build (ubuntu-latest, node 14, npm 8)	51s
build (windows-latest, node 20, npm 11)	1m 55s
build (windows-latest, node 14, npm 8)	1m 40s
build (ubuntu-latest, node 20, npm 11)	43s
build (ubuntu-latest, node 14, npm 6)	45s
	6m 19s

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