[IR] Simplify IrFileImpl and IrExternalPackageFragment

#KT-65773 In Progress

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerialInfoImplJvmIrGenerator.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2024 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -11,7 +11,6 @@ import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.builders.declarations.*
 import org.jetbrains.kotlin.ir.declarations.*
-import org.jetbrains.kotlin.ir.declarations.impl.IrExternalPackageFragmentImpl
 import org.jetbrains.kotlin.ir.declarations.impl.IrFactoryImpl
 import org.jetbrains.kotlin.ir.expressions.impl.*
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
@@ -95,7 +94,7 @@ class SerialInfoImplJvmIrGenerator(
     }
 
     private fun createPackage(packageName: String): IrPackageFragment =
-        IrExternalPackageFragmentImpl.createEmptyExternalPackageFragment(
+        createEmptyExternalPackageFragment(
             moduleFragment.descriptor,
             FqName(packageName)
         )