Prevent rebuilding by avoid registering doFirst on source tasks (KTI-324)

Do the check for empty directories at the build end instead.

Registering doFirst makes constant recompile when switching from Kotlin to Kotlin
Native composite with the reason:

```
The task was not up-to-date because of the following reasons:
One or more additional actions for task ':kotlin:kotlin-stdlib-common:compileKotlinCommon' have changed.
```

Exact reason why actions are different is not clear (there's a single
action in both builds in comparison), but probably the reason in
different types due to usage of composite build.

 #KTI-324 Fixed

build.gradle.kts

@@ -465,16 +465,6 @@ allprojects {
         outputs.doNotCacheIf("https://youtrack.jetbrains.com/issue/KT-37089") { true }
     }
 
-    tasks.withType<SourceTask>().configureEach {
-        doFirst {
-            source.visit {
-                if (file.isDirectory && file.listFiles()?.isEmpty() == true) {
-                    logger.warn("Empty source directories may cause build cache misses: " + file.absolutePath)
-                }
-            }
-        }
-    }
-
     normalization {
         runtimeClasspath {
             ignore("META-INF/MANIFEST.MF")
@@ -534,6 +524,22 @@ allprojects {
     }
 }
 
+gradle.buildFinished {
+    val taskGraph = gradle?.taskGraph
+    if (taskGraph != null) {
+        taskGraph.allTasks
+                .filterIsInstance<SourceTask>()
+                .filter { it.didWork }
+                .forEach {
+                    it.source.visit {
+                        if (file.isDirectory && file.listFiles()?.isEmpty() == true) {
+                            logger.warn("Empty source directories may cause build cache misses: " + file.absolutePath)
+                        }
+                    }
+                }
+    }
+}
+
 gradle.taskGraph.whenReady {
     fun Boolean.toOnOff(): String = if (this) "on" else "off"
     val profile = if (isTeamcityBuild) "CI" else "Local"