Enabling Kotlin incremental compilation on Buck2

What this post added

This post details the integration of Kotlin's incremental compiler into Meta's Buck2 build system. It explains the technical challenges and solutions involved, including using the Kotlin Build Tools API, managing build outputs, making the compiler cache relocatable, configuring change tracking for modules and dependencies, and adapting compiler plugins and annotation processors for incremental compilation. This effort significantly speeds up compilation times for critical modules, improving developer productivity.