[#488] Move layer/frostfs.go to layer/frostfs/frostfs.go

Signed-off-by: Nikita Zinkevich <n.zinkevich@yadro.com>

api/layer/object_test.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"testing"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/layer/frostfs"
 	"github.com/stretchr/testify/require"
 )
 
@@ -37,7 +38,7 @@ func TestGoroutinesDontLeakInPutAndHash(t *testing.T) {
 	require.NoError(t, err)
 	payload := bytes.NewReader(content)
 
-	prm := PrmObjectCreate{
+	prm := frostfs.PrmObjectCreate{
 		Filepath: tc.obj,
 		Payload:  payload,
 	}