build: split integration tests for split repositories #16069

Job	Run time
owlbot-py	59s
package_name_check	50s
owlbot-yaml	50s
gitignore_correctness	38s
group_id_check_for_maps_libraries	6m 15s
generate-readme	41s
root-pom	1m 20s
gapic-bom	1m 17s
	12m 50s

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