[BT] Add support for SourcesChanges.ToBeCalculated

[ywett02]

KT-70556

Currently, the conversion from org.jetbrains.kotlin.buildtools.api.SourcesChanges.ToBeCalculated to org.jetbrains.kotlin.incremental.ChangedFiles results in the use of ChangedFiles.Unknown (codepointer). This configuration triggers a rebuild of the whole module (codepointer).

Instead of using ChangedFiles.Unknown, I propose returning null to prompt the IC to track the file changes.

Testing

./gradlew :compiler:build-tools:kotlin-build-tools-api-tests:check

Reusing configuration cache.

BUILD SUCCESSFUL in 40s
366 actionable tasks: 163 executed, 193 from cache, 10 up-to-date

Publishing build scan...
https://gradle.com/s/eha7w77c4ygvi

Configuration cache entry reused.