Move internal packages to an internal folder

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

integration/notion/notion.go

@@ -1,54 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package notion
-
-import (
-	"fmt"
-
-	"miniflux.app/v2/http/client"
-)
-
-// Client represents a Notion client.
-type Client struct {
-	token  string
-	pageID string
-}
-
-// NewClient returns a new Notion client.
-func NewClient(token, pageID string) *Client {
-	return &Client{token, pageID}
-}
-
-func (c *Client) AddEntry(entryURL string, entryTitle string) error {
-	if c.token == "" || c.pageID == "" {
-		return fmt.Errorf("notion: missing credentials")
-	}
-	clt := client.New("https://api.notion.com/v1/blocks/" + c.pageID + "/children")
-	block := &Data{
-		Children: []Block{
-			{
-				Object: "block",
-				Type:   "bookmark",
-				Bookmark: Bookmark{
-					Caption: []interface{}{},
-					URL:     entryURL,
-				},
-			},
-		},
-	}
-	clt.WithAuthorization("Bearer " + c.token)
-	customHeaders := map[string]string{
-		"Notion-Version": "2022-06-28",
-	}
-	clt.WithCustomHeaders(customHeaders)
-	response, error := clt.PatchJSON(block)
-	if error != nil {
-		return fmt.Errorf("notion: unable to patch entry: %v", error)
-	}
-
-	if response.HasServerFailure() {
-		return fmt.Errorf("notion: request failed, status=%d", response.StatusCode)
-	}
-	return nil
-}