Make the contents of the apache plugin private (#7579)

Part of #5775.

Tree:
```
certbot-apache/certbot_apache
├── __init__.py
├── _internal
│   ├── apache_util.py
│   ├── augeas_lens
│   │   ├── httpd.aug
│   │   └── README
│   ├── centos-options-ssl-apache.conf
│   ├── configurator.py
│   ├── constants.py
│   ├── display_ops.py
│   ├── entrypoint.py
│   ├── http_01.py
│   ├── __init__.py
│   ├── obj.py
│   ├── options-ssl-apache.conf
│   ├── override_arch.py
│   ├── override_centos.py
│   ├── override_darwin.py
│   ├── override_debian.py
│   ├── override_fedora.py
│   ├── override_gentoo.py
│   ├── override_suse.py
│   └── parser.py
└── tests
    ├── ...
```

* Create _internal folder for certbot_apache

* Move apache_util.py to _internal

* Move display_ops.py to _internal

* Move override_centos.py to _internal

* Move override_gentoo.py to _internal

* Move override_darwin.py to _internal

* Move override_suse.py to _internal

* Move override_debian.py to _internal

* Move override_fedora.py to _internal

* Move override_arch.py to _internal

* Move parser.py to _internal

* Move obj.py to _internal

* Move http_01.py to _internal

* Move entrypoint.py to _internal

* Move constants.py to _internal

* Move configurator.py to _internal

* Move augeas_lens to _internal

* Move options-ssl-apache.conf files to _internal

* move augeas_lens in MANIFEST

* Clean up some stray references to certbot_apache that could use _internal

* Correct imports and lint

certbot-apache/setup.py

@@ -71,7 +71,7 @@ setup(
     install_requires=install_requires,
     entry_points={
         'certbot.plugins': [
-            'apache = certbot_apache.entrypoint:ENTRYPOINT',
+            'apache = certbot_apache._internal.entrypoint:ENTRYPOINT',
         ],
     },
     test_suite='certbot_apache',