Move internal packages to an internal folder

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

internal/locale/plural_test.go

@@ -0,0 +1,62 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package locale // import "miniflux.app/v2/internal/locale"
+
+import "testing"
+
+func TestPluralRules(t *testing.T) {
+	scenarios := map[string]map[int]int{
+		"default": {
+			1: 0,
+			2: 1,
+			5: 1,
+		},
+		"ar_AR": {
+			0:   0,
+			1:   1,
+			2:   2,
+			5:   3,
+			11:  4,
+			200: 5,
+		},
+		"cs_CZ": {
+			1: 0,
+			2: 1,
+			5: 2,
+		},
+		"pl_PL": {
+			1: 0,
+			2: 1,
+			5: 2,
+		},
+		"pt_BR": {
+			1: 0,
+			2: 1,
+			5: 1,
+		},
+		"ru_RU": {
+			1: 0,
+			2: 1,
+			5: 2,
+		},
+		"sr_RS": {
+			1: 0,
+			2: 1,
+			5: 2,
+		},
+		"zh_CN": {
+			1: 0,
+			5: 0,
+		},
+	}
+
+	for rule, values := range scenarios {
+		for input, expected := range values {
+			result := pluralForms[rule](input)
+			if result != expected {
+				t.Errorf(`Unexpected result for %q rule, got %d instead of %d for %d as input`, rule, result, expected, input)
+			}
+		}
+	}
+}