Part of #5775.
* Create _internal folder certbot-nginx
* Move configurator.py to _internal
* Move constants.py to _internal
* Move display_ops.py to _internal
* Move http_01.py to _internal
* Move nginxparser.py to _internal
* Move obj.py to _internal
* Move parser_obj.py to _internal
* Move parser.py to _internal
* Update location and references for tls_configs
* exclude parser_obj from coverage
* rewrite nginx parser to allow everything that nginx does
* also make changes in tls_sni_01.py
* add test case with
* allow embedded variables
* allow empty ${} variable
* fix quotes
* un-special case if
* update all tests to reflect current parsing
* escape in QuotedString after merge
* add test cases for variable weirdness that are almost certainly nginx bugs
* update regex for correct variable rules
* close paren doesn't invoke last_space
* Make test file valid Nginx syntax