[FIR] Move default visitors from separate sourceset

compiler/fir/tree/build.gradle.kts

@@ -18,10 +18,7 @@ dependencies {
 }
 
 sourceSets {
-    "main" {
-        projectDefault()
-        java.srcDir("visitors")
-    }
+    "main" { projectDefault() }
 }
 
 val generatorClasspath by configurations.creating

compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/CompositeTransformResult.kt

@@ -13,16 +13,12 @@ sealed class CompositeTransformResult<out T : Any>() {
 
     class Multiple<out T : Any>(val _list: List<T>) : CompositeTransformResult<T>()
 
-
     companion object {
         fun <T : Any> empty() = CompositeTransformResult.Multiple<T>(emptyList<T>())
         fun <T : Any> single(t: T) = CompositeTransformResult.Single(t)
         fun <T : Any> many(l: List<T>) = CompositeTransformResult.Multiple(l)
-
-
     }
 
-
     val list: List<T>
         get() = when (this) {
             is CompositeTransformResult.Multiple<*> -> _list as List<T>
@@ -41,8 +37,6 @@ sealed class CompositeTransformResult<out T : Any>() {
 
     val isEmpty
         get() = this is CompositeTransformResult.Multiple<*> && this.list.isEmpty()
-
-
 }
 
 @Suppress("NOTHING_TO_INLINE")