Don't show file root if it's same as the only declaration (KT-21200)

#KT-21200 Fixed

idea/idea-analysis/src/org/jetbrains/kotlin/idea/KotlinIconProvider.kt

@@ -50,6 +50,8 @@ class KotlinIconProvider : IconProvider(), DumbAware {
     companion object {
         var INSTANCE = KotlinIconProvider()
 
+        fun isSingleClassFile(file: KtFile) = getSingleClass(file) != null
+
         fun getSingleClass(file: KtFile): KtClassOrObject? {
             val mainClass = getMainClass(file)
             return if (mainClass != null && file.declarations.size == 1) mainClass else null

idea/idea-analysis/src/org/jetbrains/kotlin/idea/structureView/KotlinStructureViewFactory.java

@@ -24,6 +24,7 @@ import com.intellij.openapi.editor.Editor;
 import com.intellij.psi.PsiFile;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
+import org.jetbrains.kotlin.idea.KotlinIconProvider;
 import org.jetbrains.kotlin.psi.KtFile;
 
 public class KotlinStructureViewFactory implements PsiStructureViewFactory {
@@ -40,6 +41,11 @@ public class KotlinStructureViewFactory implements PsiStructureViewFactory {
             public StructureViewModel createStructureViewModel(@Nullable Editor editor) {
                 return new KotlinStructureViewModel(file, editor);
             }
+
+            @Override
+            public boolean isRootNodeShown() {
+                return !KotlinIconProvider.Companion.isSingleClassFile(file);
+            }
         };
     }
 }