JVM IR: do not depend on StubGeneratorExtensions in backend

Move most of the implementation from JvmGeneratorExtensions to
JvmGeneratorExtensionsImpl. The latter implements
StubGeneratorExtensions from psi2ir, and doing so helps to remove direct
dependency on this part of psi2ir from backend.

compiler/tests-common/tests/org/jetbrains/kotlin/ir/AbstractIrGeneratorTestCase.kt

@@ -18,7 +18,7 @@ package org.jetbrains.kotlin.ir
 
 import org.jetbrains.kotlin.analyzer.AnalysisResult
 import org.jetbrains.kotlin.backend.common.serialization.signature.IdSignatureDescriptor
-import org.jetbrains.kotlin.backend.jvm.JvmGeneratorExtensions
+import org.jetbrains.kotlin.backend.jvm.JvmGeneratorExtensionsImpl
 import org.jetbrains.kotlin.backend.jvm.serialization.JvmIdSignatureDescriptor
 import org.jetbrains.kotlin.cli.jvm.compiler.EnvironmentConfigFiles
 import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
@@ -141,7 +141,7 @@ abstract class AbstractIrGeneratorTestCase : CodegenTestCase() {
         ): IrModuleFragment {
             return generateIrModule(
                 JvmResolveUtil.analyze(ktFilesToAnalyze, environment), psi2ir, ktFilesToAnalyze,
-                JvmGeneratorExtensions(generateFacades = false),
+                JvmGeneratorExtensionsImpl(generateFacades = false),
                 createIdSignatureComposer = { bindingContext ->
                     JvmIdSignatureDescriptor(JvmManglerDesc(MainFunctionDetector(bindingContext, languageVersionSettings)))
                 }