Fixes to KAPT classpath change detection

1) Fix tests to use canonical path for comparison because Mac was failing
2) Update current classpath snapshot only with the missing entries from the previous one
3) Clean-up code and style

plugins/kapt3/kapt3-compiler/src/org/jetbrains/kotlin/kapt3/Kapt3Plugin.kt

@@ -101,7 +101,7 @@ class Kapt3CommandLineProcessor : CommandLineProcessor {
             COMPILED_SOURCES_DIR -> compiledSources.addAll(value.split(File.pathSeparator).map { File(it) })
             INCREMENTAL_CACHE -> incrementalCache = File(value)
             CLASSPATH_CHANGES -> classpathChanges.addAll(value.split(File.pathSeparator).map { it })
-            PROCESS_INCREMENTALLY -> processIncrementally = value.toBoolean()
+            PROCESS_INCREMENTALLY -> setFlag(KaptFlag.INCREMENTAL_APT, value)
 
             ANNOTATION_PROCESSOR_CLASSPATH_OPTION -> processingClasspath += File(value)
             ANNOTATION_PROCESSORS_OPTION -> processors.addAll(value.split(',').map { it.trim() }.filter { it.isNotEmpty() })