Move internal packages to an internal folder

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

internal/timezone/timezone.go

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package timezone // import "miniflux.app/v2/internal/timezone"
+
+import (
+	"time"
+)
+
+// Convert converts provided date time to actual timezone.
+func Convert(tz string, t time.Time) time.Time {
+	userTimezone := getLocation(tz)
+
+	if t.Location().String() == "" {
+		// In this case, the provided date is already converted to the user timezone by Postgres,
+		// but the timezone information is not set in the time struct.
+		// We cannot use time.In() because the date will be converted a second time.
+		t = time.Date(
+			t.Year(),
+			t.Month(),
+			t.Day(),
+			t.Hour(),
+			t.Minute(),
+			t.Second(),
+			t.Nanosecond(),
+			userTimezone,
+		)
+	} else if t.Location() != userTimezone {
+		t = t.In(userTimezone)
+	}
+
+	return t
+}
+
+// Now returns the current time with the given timezone.
+func Now(tz string) time.Time {
+	return time.Now().In(getLocation(tz))
+}
+
+func getLocation(tz string) *time.Location {
+	loc, err := time.LoadLocation(tz)
+	if err != nil {
+		loc = time.Local
+	}
+	return loc
+}