Move internal packages to an internal folder

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

internal/integration/instapaper/instapaper.go

@@ -0,0 +1,47 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package instapaper // import "miniflux.app/v2/internal/integration/instapaper"
+
+import (
+	"fmt"
+	"net/url"
+
+	"miniflux.app/v2/internal/http/client"
+)
+
+// Client represents an Instapaper client.
+type Client struct {
+	username string
+	password string
+}
+
+// NewClient returns a new Instapaper client.
+func NewClient(username, password string) *Client {
+	return &Client{username: username, password: password}
+}
+
+// AddURL sends a link to Instapaper.
+func (c *Client) AddURL(link, title string) error {
+	if c.username == "" || c.password == "" {
+		return fmt.Errorf("instapaper: missing credentials")
+	}
+
+	values := url.Values{}
+	values.Add("url", link)
+	values.Add("title", title)
+
+	apiURL := "https://www.instapaper.com/api/add?" + values.Encode()
+	clt := client.New(apiURL)
+	clt.WithCredentials(c.username, c.password)
+	response, err := clt.Get()
+	if err != nil {
+		return fmt.Errorf("instapaper: unable to send url: %v", err)
+	}
+
+	if response.HasServerFailure() {
+		return fmt.Errorf("instapaper: unable to send url, status=%d", response.StatusCode)
+	}
+
+	return nil
+}