Make all module_utils and plugin_utils private (#887)

* Add leading underscore. Remove deprecated module utils.

* Document module and plugin utils as private. Add changelog fragment.

* Convert relative to absolute imports.

* Remove unnecessary imports.

tests/unit/plugins/module_utils/_acme/test_io.py

@@ -0,0 +1,31 @@
+# Copyright (c) Ansible Project
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+from __future__ import annotations
+
+from unittest.mock import (
+    MagicMock,
+)
+
+from ansible_collections.community.crypto.plugins.module_utils._acme.io import (
+    read_file,
+    write_file,
+)
+
+
+TEST_TEXT = r"""1234
+5678"""
+
+
+def test_read_file(tmpdir) -> None:
+    fn = tmpdir / "test.txt"
+    fn.write(TEST_TEXT)
+    assert read_file(str(fn)) == TEST_TEXT.encode("utf-8")
+
+
+def test_write_file(tmpdir) -> None:
+    fn = tmpdir / "test.txt"
+    module = MagicMock()
+    write_file(module, str(fn), TEST_TEXT.encode("utf-8"))
+    assert fn.read() == TEST_TEXT