Improve resolution of compiled project classes

The Kotlin Debug Adapter currently relies on [hardcoded paths](https://github.com/fwcd/kotlin-debug-adapter/blob/a2a2332a081286256c790d7eb51cf0798d808fab/adapter/src/main/kotlin/org/javacs/ktda/classpath/ProjectClassesResolver.kt#L14), which is not a very robust solution. A possible solution would be to write (and invoke) a Gradle script, like we do to locate dependencies, but for [output classes](https://docs.gradle.org/current/dsl/org.gradle.api.tasks.SourceSetOutput.html).

Start with adapter/src/main/kotlin/org/javacs/ktda/classpath/ProjectClassesResolver.kt, especially its hardcoded paths, and compare it with the existing Gradle-based dependency lookup. Trace how the Kotlin Debug Adapter invokes dependency resolution. Done means compiled project output classes are resolved through the project configuration rather than fixed paths.

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