[FIR generator] Move ArbitrarilyImportable class to common module

This is a step towards commonizing the code generator between
FIR and IR: KT-61970

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/importables.kt

@@ -5,7 +5,7 @@
 
 package org.jetbrains.kotlin.fir.tree.generator
 
-import org.jetbrains.kotlin.fir.tree.generator.model.ArbitraryImportable
+import org.jetbrains.kotlin.generators.tree.ArbitraryImportable
 
 val phaseAsResolveStateExtentionImport = ArbitraryImportable("org.jetbrains.kotlin.fir.declarations", "asResolveState")
 val resolvePhaseExtensionImport = ArbitraryImportable("org.jetbrains.kotlin.fir.declarations", "resolvePhase")

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Element.kt

@@ -210,4 +210,3 @@ class TypeArgumentWithMultipleUpperBounds(name: String, override val upperBounds
     }
 }
 
-data class ArbitraryImportable(override val packageName: String, override val type: String) : Importable

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Field.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.tree.generator.model
 
 import org.jetbrains.kotlin.fir.tree.generator.printer.typeWithArguments
+import org.jetbrains.kotlin.generators.tree.ArbitraryImportable
 import org.jetbrains.kotlin.generators.tree.Importable
 
 sealed class Field : Importable {

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Implementation.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.tree.generator.model
 
+import org.jetbrains.kotlin.generators.tree.ArbitraryImportable
 import org.jetbrains.kotlin.generators.tree.Importable
 
 class ImplementationWithArg(

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/ArbitrarilyImportable.kt

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2010-2023 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.generators.tree
+
+data class ArbitraryImportable(override val packageName: String, override val type: String) : Importable