Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/packageLocalClassNotImportedWithDefaultImport.kt

@@ -0,0 +1,24 @@
+// FILE: box.kt
+
+package a
+
+import pack.*
+
+class X : SomeClass()
+
+fun box(): String {
+    X()
+    return "OK"
+}
+
+// FILE: file1.kt
+
+package kotlin.jvm
+
+private class SomeClass
+
+// FILE: file2.kt
+
+package pack
+
+public open class SomeClass