build: add helper for installing projects for a single subdirectory #16027

Job	Run time
generate-readme	56s
package_name_check	49s
group_id_check_for_maps_libraries	6m 32s
gitignore_correctness	47s
gapic-bom	1m 27s
root-pom	1m 29s
owlbot-yaml	50s
owlbot-py	57s
	13m 47s

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