Guard against duplicate classes when building grails-core

### Issue description

https://blog.gradle.org/detect-maven-hijack-risks-in-gradle-with-plugin details one possible way to guard against duplicate classes. We should explore adding this to our build for increased security.

Start by reviewing the Grails build configuration and the linked Gradle article on detecting Maven hijack risks. Determine how duplicate classes can be detected during the grails-core build, then verify that the build guards against them without breaking normal builds.

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