Notes originally from #3809:

• reuse common infrastructure like text documents
• CPD programmatic API - goal is to reuse the same abstractions as PMD like FileCollector. No compatibility story planned for master. Also see [core] Provide a CpdAnalysis class as a programmatic entry point into CPD #4204
• Related to both items, maybe the cleanest approach would be to make CPD a PMD rule, and we could remove the programmatic API of CPD (keeping the CLI for convenience)