[#19] transformer: Move `EpochSource` to other types

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

object/transformer/fmt.go

@@ -1,5 +0,0 @@
-package transformer
-
-type EpochSource interface {
-	CurrentEpoch() uint64
-}

object/transformer/types.go

@@ -16,6 +16,11 @@ type AccessIdentifiers struct {
 	ParentHeader *object.Object
 }
 
+// EpochSource is a source for the current epoch.
+type EpochSource interface {
+	CurrentEpoch() uint64
+}
+
 // ObjectTarget is an interface of the object writer.
 type ObjectTarget interface {
 	// WriteHeader writes object header w/ payload part.