chore: improve monorepo migration tooling #18730

Job	Run time
root-pom	1m 37s
gitignore_correctness	57s
owlbot-py	1m 8s
owlbot-yaml	1m 5s
package_name_check	1m 4s
group_id_check_for_maps_libraries	9m 9s
generate-readme	1m 20s
gapic-bom	1m 39s
	17m 59s

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