[FIR] Allow FirElementSerializer.packagePartProto accept multiple files

This is needed for cases when there are multiple files with same name
 and `@JvmMultifileClass` annotation

compiler/incremental-compilation-impl/src/org/jetbrains/kotlin/incremental/incrementalFirCacheUtils.kt

@@ -70,7 +70,7 @@ internal fun collectNewDirtySources(
             }
 
             override fun visitFile(file: FirFile, data: MutableList<MetadataSerializer>) {
-                val metadata = FirMetadataSource.File(file)
+                val metadata = FirMetadataSource.File(listOf(file))
                 withMetadataSerializer(metadata, data) {
                     file.acceptChildren(this, data)
                     // TODO: compare package fragments?