Move internal packages to an internal folder

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

internal/reader/parser/format.go

@@ -0,0 +1,49 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package parser // import "miniflux.app/v2/internal/reader/parser"
+
+import (
+	"encoding/xml"
+	"strings"
+
+	rxml "miniflux.app/v2/internal/reader/xml"
+)
+
+// List of feed formats.
+const (
+	FormatRDF     = "rdf"
+	FormatRSS     = "rss"
+	FormatAtom    = "atom"
+	FormatJSON    = "json"
+	FormatUnknown = "unknown"
+)
+
+// DetectFeedFormat tries to guess the feed format from input data.
+func DetectFeedFormat(data string) string {
+	if strings.HasPrefix(strings.TrimSpace(data), "{") {
+		return FormatJSON
+	}
+
+	decoder := rxml.NewDecoder(strings.NewReader(data))
+
+	for {
+		token, _ := decoder.Token()
+		if token == nil {
+			break
+		}
+
+		if element, ok := token.(xml.StartElement); ok {
+			switch element.Name.Local {
+			case "rss":
+				return FormatRSS
+			case "feed":
+				return FormatAtom
+			case "RDF":
+				return FormatRDF
+			}
+		}
+	}
+
+	return FormatUnknown
+}