Guard against duplicate classes when building grails-core

[jdaugherty]

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.

[sanjana2505006]

@jdaugherty I’ve raised PR #15341 for this. Please let me know if you have any feedback or would like any changes I’d be happy to update it.