Make the contents of the DNS plugins private (#7580)

Part of #5775.

```
modify_item () {
    mkdir certbot-dns-$1/certbot_dns_$1/_internal
    git grep -l "from certbot_dns_$1 import dns_$1" | xargs sed -i "s/from certbot_dns_$1 import dns_$1/from certbot_dns_$1._internal import dns_$1/g"
    git grep -l "certbot_dns_$1\.dns_$1" | xargs sed -i "s/certbot_dns_$1\.dns_$1/certbot_dns_$1._internal.dns_$1/g"
    git checkout -- certbot-dns-$1/certbot_dns_$1/__init__.py
    echo '"""Internal implementation of \`~certbot_dns_$1.dns_$1\` plugin."""' > certbot-dns-$1/certbot_dns_$1/_internal/__init__.py
    mv certbot-dns-$1/certbot_dns_$1/dns_$1.py certbot-dns-$1/certbot_dns_$1/_internal
    git checkout -- CHANGELOG.md
    git status
    git add -A
    git commit -m "Move certbot-dns-$1 to _internal structure"
}
```

Structure now looks like this:
```
certbot-dns-cloudflare/
├── certbot_dns_cloudflare
│   ├── dns_cloudflare_test.py
│   ├── __init__.py
│   └── _internal
│       ├── dns_cloudflare.py
│       └── __init__.py
```

* Move certbot-dns-cloudflare to _internal structure

* Move certbot-dns-cloudxns to _internal structure

* Move certbot-dns-digitalocean to _internal structure

* Move certbot-dns-dnsimple to _internal structure

* Move certbot-dns-dnsmadeeasy to _internal structure

* Move certbot-dns-gehirn to _internal structure

* Move certbot-dns-google to _internal structure

* Move certbot-dns-linode to _internal structure

* Move certbot-dns-luadns to _internal structure

* Move certbot-dns-nsone to _internal structure

* Move certbot-dns-ovh to _internal structure

* Move certbot-dns-rfc2136 to _internal structure

* Move certbot-dns-sakuracloud to _internal structure

* Init file comments need to be comments

* Move certbot-dns-route53 to _internal structure

* Fix comment in route53 init

certbot-dns-nsone/certbot_dns_nsone/_internal/__init__.py

@@ -0,0 +1 @@
+"""Internal implementation of `~certbot_dns_nsone.dns_nsone` plugin."""

certbot-dns-nsone/certbot_dns_nsone/dns_nsone_test.py

@@ -1,4 +1,4 @@
-"""Tests for certbot_dns_nsone.dns_nsone."""
+"""Tests for certbot_dns_nsone._internal.dns_nsone."""
 
 import unittest
 
@@ -20,7 +20,7 @@ class AuthenticatorTest(test_util.TempDirTestCase,
     def setUp(self):
         super(AuthenticatorTest, self).setUp()
 
-        from certbot_dns_nsone.dns_nsone import Authenticator
+        from certbot_dns_nsone._internal.dns_nsone import Authenticator
 
         path = os.path.join(self.tempdir, 'file.ini')
         dns_test_common.write({"nsone_api_key": API_KEY}, path)
@@ -40,7 +40,7 @@ class NS1LexiconClientTest(unittest.TestCase, dns_test_common_lexicon.BaseLexico
     LOGIN_ERROR = HTTPError('401 Client Error: Unauthorized for url: {0}.'.format(DOMAIN))
 
     def setUp(self):
-        from certbot_dns_nsone.dns_nsone import _NS1LexiconClient
+        from certbot_dns_nsone._internal.dns_nsone import _NS1LexiconClient
 
         self.client = _NS1LexiconClient(API_KEY, 0)
 

certbot-dns-nsone/setup.py

@@ -60,7 +60,7 @@ setup(
     },
     entry_points={
         'certbot.plugins': [
-            'dns-nsone = certbot_dns_nsone.dns_nsone:Authenticator',
+            'dns-nsone = certbot_dns_nsone._internal.dns_nsone:Authenticator',
         ],
     },
     test_suite='certbot_dns_nsone',