[FIR] Extract FirPluginKey into :core:compiler.common and rename it to GeneratedDeclarationKey

This is needed to avoid dependency on :compiler:fir:fir2ir module in
  backend parts of compiler plugins. It will allow to publish those
  parts into jars for IDE, where they are needed for working of debugger
  and bytecode toolwindow

core/compiler.common/src/org/jetbrains/kotlin/GeneratedDeclarationKey.kt

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2010-2022 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.
+ */
+
+package org.jetbrains.kotlin
+
+abstract class GeneratedDeclarationKey