[FIR] Replace Path with String in KotlinJvmBinaryClass.containingLibrary

This is needed because :core:deserialization.common.jvm uses JDK 6

compiler/frontend.java/src/org/jetbrains/kotlin/load/kotlin/VirtualFileKotlinClass.kt

@@ -26,8 +26,6 @@ import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.util.PerformanceCounter
 import java.io.FileNotFoundException
 import java.io.IOException
-import java.nio.file.Path
-import java.nio.file.Paths
 
 class VirtualFileKotlinClass private constructor(
     val file: VirtualFile,
@@ -40,8 +38,8 @@ class VirtualFileKotlinClass private constructor(
     override val location: String
         get() = file.path
 
-    override val containingLibrary: Path?
-        get() = file.path.split("!/").firstOrNull()?.let { Paths.get(it) }
+    override val containingLibrary: String?
+        get() = file.path.split("!/").firstOrNull()
 
     override fun getFileContents(): ByteArray {
         try {