Move internal packages to an internal folder

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

internal/url/url.go

@@ -0,0 +1,81 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package url // import "miniflux.app/v2/internal/url"
+
+import (
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+// IsAbsoluteURL returns true if the link is absolute.
+func IsAbsoluteURL(link string) bool {
+	u, err := url.Parse(link)
+	if err != nil {
+		return false
+	}
+	return u.IsAbs()
+}
+
+// AbsoluteURL converts the input URL as absolute URL if necessary.
+func AbsoluteURL(baseURL, input string) (string, error) {
+	if strings.HasPrefix(input, "//") {
+		input = "https://" + input[2:]
+	}
+
+	u, err := url.Parse(input)
+	if err != nil {
+		return "", fmt.Errorf("unable to parse input URL: %v", err)
+	}
+
+	if u.IsAbs() {
+		return u.String(), nil
+	}
+
+	base, err := url.Parse(baseURL)
+	if err != nil {
+		return "", fmt.Errorf("unable to parse base URL: %v", err)
+	}
+
+	return base.ResolveReference(u).String(), nil
+}
+
+// RootURL returns absolute URL without the path.
+func RootURL(websiteURL string) string {
+	if strings.HasPrefix(websiteURL, "//") {
+		websiteURL = "https://" + websiteURL[2:]
+	}
+
+	absoluteURL, err := AbsoluteURL(websiteURL, "")
+	if err != nil {
+		return websiteURL
+	}
+
+	u, err := url.Parse(absoluteURL)
+	if err != nil {
+		return absoluteURL
+	}
+
+	return u.Scheme + "://" + u.Host + "/"
+}
+
+// IsHTTPS returns true if the URL is using HTTPS.
+func IsHTTPS(websiteURL string) bool {
+	parsedURL, err := url.Parse(websiteURL)
+	if err != nil {
+		return false
+	}
+
+	return strings.ToLower(parsedURL.Scheme) == "https"
+}
+
+// Domain returns only the domain part of the given URL.
+func Domain(websiteURL string) string {
+	parsedURL, err := url.Parse(websiteURL)
+	if err != nil {
+		return websiteURL
+	}
+
+	return parsedURL.Host
+}