[FIR generator] Move Importable interface to common module

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

generators/tree-generator-common/ReadMe.md

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+## Common code for the FIR and IR tree generators
+
+The [FIR](../../compiler/fir/tree/tree-generator) and [IR](../../compiler/ir/ir.tree/tree-generator) tree generators, although quite
+different, share some common logic. This logic is stored here in this module.

generators/tree-generator-common/build.gradle.kts

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+plugins {
+    kotlin("jvm")
+    id("jps-compatible")
+}
+
+dependencies {
+    implementation(project(":core:compiler.common"))
+}
+
+sourceSets {
+    "main" {
+        projectDefault()
+    }
+    "test" {}
+}

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

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+/*
+ * Copyright 2010-2019 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
+
+interface Importable {
+    val type: String
+    val packageName: String?
+    val fullQualifiedName: String? get() = packageName?.let { "$it.${type.replace(Regex("<.+>"), "")}" }
+}