[FIR/IR generator] Introduce ImportCollector

This class enables printing the import list in generated files
in a smarter way.

Also, refactor `Importable` interface hierarchy, namely, don't inherit
`TypeRef` from `Importable`, since we have types like `TypeRef.Star`
which are not really importable.

Replace the `Importable#typeWithArguments` property with
the `TypeRef#render` method to utilize `ImportCollector` while rendering
types.

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/config/ConfigModel.kt

@@ -10,7 +10,6 @@ import com.squareup.kotlinpoet.PropertySpec
 import com.squareup.kotlinpoet.TypeSpec
 import org.jetbrains.kotlin.generators.tree.*
 import org.jetbrains.kotlin.ir.generator.BASE_PACKAGE
-import org.jetbrains.kotlin.ir.generator.model.Element
 import org.jetbrains.kotlin.ir.generator.util.*
 
 class Config(
@@ -74,11 +73,10 @@ class ElementConfig(
 
     override fun toString() = element.name
 
-    override val packageName: String
-        get() = category.packageName
-
-    override val type: String
-        get() = Element.elementName2typeName(name)
+    context(ImportCollector)
+    override fun renderTo(appendable: Appendable) {
+        renderingIsNotSupported()
+    }
 
     enum class Category(private val packageDir: String, val defaultVisitorParam: String) {
         Expression("expressions", "expression"),
@@ -103,11 +101,10 @@ class ElementConfigRef(
 
     override fun toString() = element.name
 
-    override val type: String
-        get() = element.type
-
-    override val packageName: String
-        get() = element.packageName
+    context(ImportCollector)
+    override fun renderTo(appendable: Appendable) {
+        renderingIsNotSupported()
+    }
 }
 
 sealed class UseFieldAsParameterInIrFactoryStrategy {

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/model/Model.kt

@@ -46,12 +46,9 @@ class Element(
         null -> null
     }
 
-    val typeName
+    override val typeName
         get() = elementName2typeName(name)
 
-    override val type: String
-        get() = typeName
-
     var isLeaf = config.isForcedLeaf
     val childrenOrderOverride: List<String>? = config.childrenOrderOverride
     override var walkableChildren: List<Field> = emptyList()

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/model/Transformations.kt

@@ -14,13 +14,11 @@ import org.jetbrains.kotlin.utils.addToStdlib.partitionIsInstance
 import org.jetbrains.kotlin.generators.tree.ElementRef as GenericElementRef
 
 private object InferredOverriddenType : TypeRefWithNullability {
-    override val type: String
-        get() = error("not supported")
-    override val packageName: String?
-        get() = null
 
-    override val typeWithArguments: String
-        get() = error("not supported")
+    context(ImportCollector)
+    override fun renderTo(appendable: Appendable) {
+        renderingIsNotSupported()
+    }
 
     override fun substitute(map: TypeParameterSubstitutionMap) = this
 

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/print/Types.kt

@@ -14,14 +14,14 @@ import org.jetbrains.kotlin.ir.generator.util.*
 import org.jetbrains.kotlin.types.Variance
 import  org.jetbrains.kotlin.generators.tree.ElementRef as GenericElementRef
 
-fun Element.toPoet() = ClassName(packageName, typeName)
+fun Element.toPoet() = ClassName(packageName, this.typeName)
 fun Element.toPoetSelfParameterized() = toPoet().parameterizedByIfAny(poetTypeVariables)
 fun Element.toPoetStarParameterized() = toPoet().parameterizedByIfAny(List(params.size) { STAR })
 val Element.poetTypeVariables get() = params.map { TypeVariableName(it.name) }
 
 fun TypeRef.toPoet(): TypeName {
     return when (this) {
-        is GenericElementRef<*, *> -> ClassName(element.packageName!!, element.type).parameterizedByIfAny(typeArgsToPoet())
+        is GenericElementRef<*, *> -> ClassName(element.packageName, element.typeName).parameterizedByIfAny(typeArgsToPoet())
         is ClassRef<*> -> ClassName(packageName, simpleNames).parameterizedByIfAny(typeArgsToPoet())
         is NamedTypeParameterRef -> TypeVariableName(name)
         is ElementConfigOrRef -> ClassName(this.element.category.packageName, elementName2typeName(element.name)).parameterizedByIfAny(