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