Move internal packages to an internal folder

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

model/integration.go

@@ -1,60 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package model // import "miniflux.app/v2/model"
-
-// Integration represents user integration settings.
-type Integration struct {
-	UserID               int64
-	PinboardEnabled      bool
-	PinboardToken        string
-	PinboardTags         string
-	PinboardMarkAsUnread bool
-	InstapaperEnabled    bool
-	InstapaperUsername   string
-	InstapaperPassword   string
-	FeverEnabled         bool
-	FeverUsername        string
-	FeverToken           string
-	GoogleReaderEnabled  bool
-	GoogleReaderUsername string
-	GoogleReaderPassword string
-	WallabagEnabled      bool
-	WallabagOnlyURL      bool
-	WallabagURL          string
-	WallabagClientID     string
-	WallabagClientSecret string
-	WallabagUsername     string
-	WallabagPassword     string
-	NunuxKeeperEnabled   bool
-	NunuxKeeperURL       string
-	NunuxKeeperAPIKey    string
-	NotionEnabled        bool
-	NotionToken          string
-	NotionPageID         string
-	EspialEnabled        bool
-	EspialURL            string
-	EspialAPIKey         string
-	EspialTags           string
-	ReadwiseEnabled      bool
-	ReadwiseAPIKey       string
-	PocketEnabled        bool
-	PocketAccessToken    string
-	PocketConsumerKey    string
-	TelegramBotEnabled   bool
-	TelegramBotToken     string
-	TelegramBotChatID    string
-	LinkdingEnabled      bool
-	LinkdingURL          string
-	LinkdingAPIKey       string
-	LinkdingTags         string
-	LinkdingMarkAsUnread bool
-	MatrixBotEnabled     bool
-	MatrixBotUser        string
-	MatrixBotPassword    string
-	MatrixBotURL         string
-	MatrixBotChatID      string
-	AppriseEnabled       bool
-	AppriseURL           string
-	AppriseServicesURL   string
-}