Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

api/payload.go

@@ -1,23 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package api // import "miniflux.app/v2/api"
-
-import (
-	"miniflux.app/v2/model"
-)
-
-type feedIconResponse struct {
-	ID       int64  `json:"id"`
-	MimeType string `json:"mime_type"`
-	Data     string `json:"data"`
-}
-
-type entriesResponse struct {
-	Total   int           `json:"total"`
-	Entries model.Entries `json:"entries"`
-}
-
-type feedCreationResponse struct {
-	FeedID int64 `json:"feed_id"`
-}