Use "pack file" instead of "data file" (#2885)

- changed variable names, especially changed DataFile into PackFile
- changed in some comments
- always use "pack file" in docu

internal/backend/layout_s3legacy.go

@@ -11,7 +11,7 @@ type S3LegacyLayout struct {
 }
 
 var s3LayoutPaths = map[restic.FileType]string{
-	restic.DataFile:     "data",
+	restic.PackFile:     "data",
 	restic.SnapshotFile: "snapshot",
 	restic.IndexFile:    "index",
 	restic.LockFile:     "lock",