IR: introduce StubGeneratorExtensions for ExternalDependenciesGenerator

This is a refactoring to simplify the API of
ExternalDependenciesGenerator and to constrain future additions of
platform-specific stub generator extensions.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt

@@ -6,7 +6,6 @@
 package org.jetbrains.kotlin.ir.backend.js
 
 import com.intellij.openapi.project.Project
-import org.jetbrains.kotlin.library.resolver.KotlinLibraryResolveResult
 import org.jetbrains.kotlin.backend.common.phaser.PhaseConfig
 import org.jetbrains.kotlin.backend.common.phaser.invokeToplevel
 import org.jetbrains.kotlin.config.CompilerConfiguration
@@ -18,6 +17,7 @@ import org.jetbrains.kotlin.ir.declarations.IrModuleFragment
 import org.jetbrains.kotlin.ir.util.ExternalDependenciesGenerator
 import org.jetbrains.kotlin.ir.util.patchDeclarationParents
 import org.jetbrains.kotlin.library.KotlinLibrary
+import org.jetbrains.kotlin.library.resolver.KotlinLibraryResolveResult
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.utils.DFS
@@ -57,12 +57,7 @@ fun compile(
 
     // Load declarations referenced during `context` initialization
     dependencyModules.forEach {
-        ExternalDependenciesGenerator(
-            it.descriptor,
-            symbolTable,
-            irBuiltIns,
-            deserializer = deserializer
-        ).generateUnboundSymbolsAsDependencies()
+        ExternalDependenciesGenerator(it.descriptor, symbolTable, irBuiltIns, deserializer).generateUnboundSymbolsAsDependencies()
     }
 
     val irFiles = dependencyModules.flatMap { it.files } + moduleFragment.files
@@ -71,11 +66,7 @@ fun compile(
     moduleFragment.files += irFiles
 
     // Create stubs
-    ExternalDependenciesGenerator(
-        moduleDescriptor = moduleDescriptor,
-        symbolTable = symbolTable,
-        irBuiltIns = irBuiltIns
-    ).generateUnboundSymbolsAsDependencies()
+    ExternalDependenciesGenerator(moduleDescriptor, symbolTable, irBuiltIns).generateUnboundSymbolsAsDependencies()
     moduleFragment.patchDeclarationParents()
 
     moveBodilessDeclarationsToSeparatePlace(context, moduleFragment)