Move internal packages to an internal folder

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

internal/reader/atom/atom_common.go

@@ -0,0 +1,83 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package atom // import "miniflux.app/v2/internal/reader/atom"
+
+import "strings"
+
+type atomPerson struct {
+	Name  string `xml:"name"`
+	Email string `xml:"email"`
+}
+
+func (a *atomPerson) String() string {
+	name := ""
+
+	switch {
+	case a.Name != "":
+		name = a.Name
+	case a.Email != "":
+		name = a.Email
+	}
+
+	return strings.TrimSpace(name)
+}
+
+type atomAuthors []*atomPerson
+
+func (a atomAuthors) String() string {
+	var authors []string
+
+	for _, person := range a {
+		authors = append(authors, person.String())
+	}
+
+	return strings.Join(authors, ", ")
+}
+
+type atomLink struct {
+	URL    string `xml:"href,attr"`
+	Type   string `xml:"type,attr"`
+	Rel    string `xml:"rel,attr"`
+	Length string `xml:"length,attr"`
+}
+
+type atomLinks []*atomLink
+
+func (a atomLinks) originalLink() string {
+	for _, link := range a {
+		if strings.ToLower(link.Rel) == "alternate" {
+			return strings.TrimSpace(link.URL)
+		}
+
+		if link.Rel == "" && (link.Type == "" || link.Type == "text/html") {
+			return strings.TrimSpace(link.URL)
+		}
+	}
+
+	return ""
+}
+
+func (a atomLinks) firstLinkWithRelation(relation string) string {
+	for _, link := range a {
+		if strings.ToLower(link.Rel) == relation {
+			return strings.TrimSpace(link.URL)
+		}
+	}
+
+	return ""
+}
+
+func (a atomLinks) firstLinkWithRelationAndType(relation string, contentTypes ...string) string {
+	for _, link := range a {
+		if strings.ToLower(link.Rel) == relation {
+			for _, contentType := range contentTypes {
+				if strings.ToLower(link.Type) == contentType {
+					return strings.TrimSpace(link.URL)
+				}
+			}
+		}
+	}
+
+	return ""
+}