FIR IDE: implement FirIdeProvider.getClassNamesInPackage

#KT-41048 fixed

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/IndexHelper.kt

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.idea.fir.low.level.api.IndexHelper.Companion.asStrin
 import org.jetbrains.kotlin.idea.stubindex.*
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtFunction
 import org.jetbrains.kotlin.psi.KtNamedFunction
 import org.jetbrains.kotlin.psi.KtProperty
@@ -66,6 +67,11 @@ public class IndexHelper(val project: Project, private val scope: GlobalSearchSc
     fun getTopLevelFunctionsInPackage(packageFqName: FqName): Collection<KtFunction> =
         KotlinTopLevelFunctionByPackageIndex.getInstance().get(packageFqName.asStringForIndexes(), project, scope)
 
+    fun getClassNamesInPackage(packageFqName: FqName): Set<Name> =
+        KotlinTopLevelClassByPackageIndex.getInstance()
+            .get(packageFqName.asStringForIndexes(), project, scope)
+            .mapNotNullTo(hashSetOf()) { it.nameAsName }
+
 
     companion object {
         private fun CallableId.asStringForIndexes(): String =

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/providers/FirIdeProvider.kt

@@ -114,10 +114,8 @@ internal class FirIdeProvider(
         TODO()
     }
 
-    override fun getClassNamesInPackage(fqName: FqName): Set<Name> {
-        // TODO: KT-41048
-        return emptySet()
-    }
+    override fun getClassNamesInPackage(fqName: FqName): Set<Name> =
+        indexHelper.getClassNamesInPackage(fqName)
 
     @NoMutableState
     private inner class SymbolProvider : FirSymbolProvider(session) {