enforce-linkage-checker

[elharo]

Set up the enforce-linkage-checker rule in pom.xml

https://github.com/GoogleCloudPlatform/cloud-opensource-java/tree/master/enforcer-rules

[athakor]

@elharo do you want to add enforcer-rules in each client pom.xml?

[elharo]

We want linkage checks enforced across the projects. If that can be done ina single pom.xml file, that's good. If not, we can put it in the individual pom.xml's.