[FIR] Move base FirExtension infrastructure to :fir:tree module

This is needed for type attribute extension which is used not only
  from :resolve module but also from :(de)serialization modules

compiler/fir/tree/src/org/jetbrains/kotlin/fir/extensions/FirExtension.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 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.
  */
 
@@ -26,7 +26,7 @@ abstract class FirExtension(val session: FirSession) {
 
     abstract val key: FirPluginKey
 
-    internal abstract val extensionType: KClass<out FirExtension>
+    abstract val extensionType: KClass<out FirExtension>
 
     fun interface Factory<out P : FirExtension> {
         fun create(session: FirSession): P

compiler/fir/tree/src/org/jetbrains/kotlin/fir/extensions/predicate/DeclarationPredicate.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 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.
  */
 
@@ -13,7 +13,7 @@ sealed class DeclarationPredicate {
     abstract val annotations: Set<AnnotationFqn>
     abstract val metaAnnotations: Set<AnnotationFqn>
 
-    internal abstract fun <R, D> accept(visitor: DeclarationPredicateVisitor<R, D>, data: D): R
+    abstract fun <R, D> accept(visitor: DeclarationPredicateVisitor<R, D>, data: D): R
 
     object Any : DeclarationPredicate() {
         override val annotations: Set<AnnotationFqn>

compiler/fir/tree/src/org/jetbrains/kotlin/fir/extensions/predicate/DeclarationPredicateVisitor.kt

@@ -1,11 +1,11 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 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.fir.extensions.predicate
 
-internal abstract class DeclarationPredicateVisitor<R, D> {
+abstract class DeclarationPredicateVisitor<R, D> {
     abstract fun visitPredicate(predicate: DeclarationPredicate, data: D): R
     open fun visitAny(predicate: DeclarationPredicate.Any, data: D): R {
         return visitPredicate(predicate, data)
@@ -42,4 +42,4 @@ internal abstract class DeclarationPredicateVisitor<R, D> {
     open fun visitUnderMetaAnnotated(predicate: UnderMetaAnnotated, data: D): R {
         return visitMetaAnnotated(predicate, data)
     }
-}
+}