fix(generator): use json_name when available #18280

Job	Run time
generate-readme	1m 19s
owlbot-py	1m 14s
root-pom	1m 46s
gitignore_correctness	1m 3s
gapic-bom	2m 53s
owlbot-yaml	1m 5s
group_id_check_for_maps_libraries	8m 49s
package_name_check	1m 13s
	19m 22s

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