Move internal packages to an internal folder

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

internal/model/model.go

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+// OptionalString populates an optional string field.
+func OptionalString(value string) *string {
+	if value != "" {
+		return &value
+	}
+	return nil
+}
+
+// OptionalInt populates an optional int field.
+func OptionalInt(value int) *int {
+	if value > 0 {
+		return &value
+	}
+	return nil
+}
+
+// OptionalInt64 populates an optional int64 field.
+func OptionalInt64(value int64) *int64 {
+	if value > 0 {
+		return &value
+	}
+	return nil
+}