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/cache/cache.go

@@ -50,7 +50,7 @@ const cacheVersion = 1
 var _ restic.Cache = &Cache{}
 
 var cacheLayoutPaths = map[restic.FileType]string{
-	restic.DataFile:     "data",
+	restic.PackFile:     "data",
 	restic.SnapshotFile: "snapshots",
 	restic.IndexFile:    "index",
 }

internal/cache/file_test.go

@@ -88,7 +88,7 @@ func TestFiles(t *testing.T) {
 
 	var tests = []restic.FileType{
 		restic.SnapshotFile,
-		restic.DataFile,
+		restic.PackFile,
 		restic.IndexFile,
 	}
 
@@ -144,7 +144,7 @@ func TestFileSaveWriter(t *testing.T) {
 	id := restic.ID{}
 	copy(id[:], data)
 	h := restic.Handle{
-		Type: restic.DataFile,
+		Type: restic.PackFile,
 		Name: id.String(),
 	}
 
@@ -202,7 +202,7 @@ func TestFileLoad(t *testing.T) {
 	id := restic.ID{}
 	copy(id[:], data)
 	h := restic.Handle{
-		Type: restic.DataFile,
+		Type: restic.PackFile,
 		Name: id.String(),
 	}
 	if err := c.Save(h, bytes.NewReader(data)); err != nil {