KT-45777: Compute classpath changes based on changed snapshots only

to avoid unnecessarily loading unchanged ones.

Duplicate classes will make this a bit tricky. This commit outlines the
algorithm to handle them, the full implementation will follow later.

Also handle removed classes when computing classpath changes.

Test: New tests in ClasspathChangesComputerTest

build-common/src/org/jetbrains/kotlin/incremental/AbstractIncrementalCache.kt

@@ -109,15 +109,17 @@ abstract class AbstractIncrementalCache<ClassName>(
 
     override fun markDirty(removedAndCompiledSources: Collection<File>) {
         for (sourceFile in removedAndCompiledSources) {
-            val classes = sourceToClassesMap[sourceFile]
-            classes.forEach {
-                dirtyOutputClassesMap.markDirty(it)
+            sourceToClassesMap[sourceFile].forEach { className ->
+                markDirty(className)
             }
-
             sourceToClassesMap.clearOutputsForSource(sourceFile)
         }
     }
 
+    fun markDirty(className: ClassName) {
+        dirtyOutputClassesMap.markDirty(className)
+    }
+
     /**
      * Updates class storage based on the given class proto.
      *