chore: improve monorepo migration tooling #18718

Job	Run time
package_name_check	58s
generate-readme	1m 14s
gapic-bom	1m 51s
owlbot-py	1m 35s
gitignore_correctness	1m 3s
root-pom	1m 39s
group_id_check_for_maps_libraries	8m 46s
owlbot-yaml	1m 3s
	18m 9s

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