merge certbot-apache and certbot-nginx into certbot (#10522)

based on the suggestion @bmw made in #10484, this moves nearly
everything from `certbot-apache` and `certbot-nginx` into subdirectories
in `certbot/src/certbot/_internal`, and corresponding "extra"
dependencies are made for the certbot distribution. in their place,
entrypoint shims are made in the old distributions.

this way, installing `certbot[nginx]` will pull in the extra
dependencies needed for the nginx code, and also pull in the shim in
`certbot-nginx`, letting our plugin discovery system work just as it did
before. ditto for apache.

note that this doesn't yet deprecate anything, which was one of the
primary goals of the original issue -- i spun out that work into #10521

fixes #10484

---------

Co-authored-by: Brad Warren <bmw@users.noreply.github.com>
Co-authored-by: ohemorange <erica@eff.org>

certbot-compatibility-test/nginx/README

@@ -4,7 +4,7 @@ Right now, this is data for the roundtrip test (checking that the parser
 can parse each file and that the reserialized config file it generates is
 identical to the original).
 
-If run in a virtualenv or otherwise so that certbot_nginx can be imported,
+If run in a virtualenv or otherwise so that certbot can be imported,
 the roundtrip test can run as
 
 python roundtrip.py nginx-roundtrip-testdata

certbot-compatibility-test/nginx/roundtrip.py

@@ -3,7 +3,7 @@
 import os
 import sys
 
-from certbot_nginx._internal import nginxparser
+from certbot._internal.plugins.nginx import nginxparser
 
 
 def roundtrip(stuff):