Move internal packages to an internal folder

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

model/api_key.go

@@ -1,32 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package model // import "miniflux.app/v2/model"
-
-import (
-	"time"
-
-	"miniflux.app/v2/crypto"
-)
-
-// APIKey represents an application API key.
-type APIKey struct {
-	ID          int64
-	UserID      int64
-	Token       string
-	Description string
-	LastUsedAt  *time.Time
-	CreatedAt   time.Time
-}
-
-// NewAPIKey initializes a new APIKey.
-func NewAPIKey(userID int64, description string) *APIKey {
-	return &APIKey{
-		UserID:      userID,
-		Token:       crypto.GenerateRandomString(32),
-		Description: description,
-	}
-}
-
-// APIKeys represents a collection of API Key.
-type APIKeys []*APIKey