Move: Fix incorrect FqName when moving declaration to default package

#KT-15586 Fixed

idea/src/org/jetbrains/kotlin/idea/refactoring/move/moveUtils.kt

@@ -188,7 +188,8 @@ fun KtElement.lazilyProcessInternalReferencesToUpdateOnPackageNameChange(
                     val prefix = containerFqName.asString()
                     val prefixOffset = it.indexOf(prefix)
                     val newFqName = if (prefix.isEmpty()) {
-                        FqName("${newContainer.fqName!!.asString()}.$it")
+                        val newContainerFqName = newContainer.fqName!!
+                        FqName(if (newContainerFqName.isRoot) it else "${newContainer.fqName!!.asString()}.$it")
                     }
                     else {
                         FqName(it.replaceRange(prefixOffset..prefixOffset + prefix.length - 1, newContainer.fqName!!.asString()))

idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/after/a.kt

@@ -0,0 +1,6 @@
+class a {
+    companion object b {
+        fun c() = "..."
+    }
+    fun d() = c()
+}

idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/after/test.kt

@@ -0,0 +1,3 @@
+fun foo() {
+
+}

idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/before/test.kt

@@ -0,0 +1,10 @@
+class <caret>a {
+    companion object b {
+        fun c() = "..."
+    }
+    fun d() = c()
+}
+
+fun foo() {
+
+}

idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/moveClassToFileInDefaultPackage.test

@@ -0,0 +1,4 @@
+{
+  "mainFile": "test.kt",
+  "intentionClass": "org.jetbrains.kotlin.idea.refactoring.move.moveDeclarations.MoveDeclarationToSeparateFileIntention"
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/MultiFileIntentionTestGenerated.java

@@ -78,6 +78,12 @@ public class MultiFileIntentionTestGenerated extends AbstractMultiFileIntentionT
         doTest(fileName);
     }
 
+    @TestMetadata("moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/moveClassToFileInDefaultPackage.test")
+    public void testMoveDeclarationToSeparateFile_moveClassToFileInDefaultPackage_MoveClassToFileInDefaultPackage() throws Exception {
+        String fileName = KotlinTestUtils.navigationMetadata("idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveClassToFileInDefaultPackage/moveClassToFileInDefaultPackage.test");
+        doTest(fileName);
+    }
+
     @TestMetadata("moveDeclarationToSeparateFile/moveSingleToFile/moveSingleToFile.test")
     public void testMoveDeclarationToSeparateFile_moveSingleToFile_MoveSingleToFile() throws Exception {
         String fileName = KotlinTestUtils.navigationMetadata("idea/testData/multiFileIntentions/moveDeclarationToSeparateFile/moveSingleToFile/moveSingleToFile.test");