KT-45777: Shrink classpath snapshot incrementally

Currently, we shrink classpath snapshots at 2 steps:
  - Classpath diffing: Shrink the current classpath snapshot against
    the previous lookup symbols
  - Classpath snapshot saving: Shrink the current classpath snapshot
    against the current lookup symbols

With this commit, the shrinking at the second step is now incremental.
The shrinking at the first step is still non-incremental.

compiler/incremental-compilation-impl/test/org/jetbrains/kotlin/incremental/classpathDiff/ClasspathChangesComputerTest.kt

@@ -293,7 +293,7 @@ private fun snapshotClasspath(classpathSourceDir: File, tmpDir: TemporaryFolder,
         classpath.addAll(listOfNotNull(classFiles.firstOrNull()?.classRoot))
 
         val relativePaths = classFiles.map { it.unixStyleRelativePath }
-        val classSnapshots = classFiles.snapshotAll(protoBased).map { it.addHash() }
+        val classSnapshots = classFiles.snapshotAll(protoBased).map { it.withHash }
         ClasspathEntrySnapshot(
             classSnapshots = relativePaths.zip(classSnapshots).toMap(LinkedHashMap())
         )