[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

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationOrigin.kt

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlin.fir.declarations
 
+import org.jetbrains.kotlin.GeneratedDeclarationKey
+
 sealed class FirDeclarationOrigin(
     private val displayName: String? = null,
     val fromSupertypes: Boolean = false,
@@ -31,13 +33,12 @@ sealed class FirDeclarationOrigin(
     object RenamedForOverride : FirDeclarationOrigin()
     object WrappedIntegerOperator : FirDeclarationOrigin()
 
-    class Plugin(val key: FirPluginKey) : FirDeclarationOrigin(displayName = "Plugin[$key]", generated = true)
+    class Plugin(val key: GeneratedDeclarationKey) : FirDeclarationOrigin(displayName = "Plugin[$key]", generated = true)
 
     override fun toString(): String {
         return displayName ?: this::class.simpleName!!
     }
 }
 
-abstract class FirPluginKey {
-    val origin: FirDeclarationOrigin = FirDeclarationOrigin.Plugin(this)
-}
+val GeneratedDeclarationKey.origin: FirDeclarationOrigin
+    get() = FirDeclarationOrigin.Plugin(this)