Implement a consistent realpath function in certbot.compat.filesystem (#7242)

Fixes #7115 

This PR creates a `realpath` method in `filesystem`, whose goal is to replace any call to `os.path.realpath` in Certbot. The reason is that `os.path.realpath` is broken on some versions of Python for Windows. See https://bugs.python.org/issue9949. The function created here works consistently across Linux and Windows.

As for the other forbidden functions in `os` module, our `certbot.compat.os` will raise an exception if its `path.realpath` function is invoked, and using the `os` module from Python is forbidden from the pylint check implemented in our CI.

Every call to `os.path.realpath` is corrected in `certbot` and `certbot-apache` modules.

* Forbid os.path.realpath

* Finish implementation

* Use filesystem.realpath

* Control symlink loops also for Linux

* Add a test for forbidden method

* Import a new object from os.path module

* Use same approach of wrapping than certbot.compat.os

* Correct errors

* Fix dependencies

* Make path module internal

certbot/plugins/common.py

@@ -486,7 +486,7 @@ def dir_setup(test_dir, pkg):  # pragma: no cover
         link, (ex: OS X) such plugins will be confused. This function prevents
         such a case.
         """
-        return os.path.realpath(tempfile.mkdtemp(prefix))
+        return filesystem.realpath(tempfile.mkdtemp(prefix))
 
     temp_dir = expanded_tempdir("temp")
     config_dir = expanded_tempdir("config")

certbot/plugins/storage_test.py

@@ -31,7 +31,7 @@ class PluginStorageTest(test_util.ConfigTestCase):
         self.plugin.storage.storagepath = os.path.join(self.config.config_dir,
                                                        ".pluginstorage.json")
         with mock.patch("six.moves.builtins.open", mock_open):
-            with mock.patch('os.path.isfile', return_value=True):
+            with mock.patch('certbot.compat.os.path.isfile', return_value=True):
                 with mock.patch("certbot.reverter.util"):
                     self.assertRaises(errors.PluginStorageError,
                                       self.plugin.storage._load)  # pylint: disable=protected-access