Fixes#8103.
* Update the DNS plugin generator script to core20 syntax
* Generate new snapcraft.yamls for the DNS plugins
* Update certbot.wrapper to search for python3.8 paths
This will allow DNS plugin snaps to build if they rely on unreleased acme/certbot, and remove other copy of Certbot from externally snapped plugins. Fixes#8064 and fixes#7946. Implementation is based on the design [here](https://github.com/certbot/certbot/issues/8064#issuecomment-645513120).
To test, see reverted commit 8632064. Steps taken:
- added changes to setup.py and snapcraft.yaml
- successfully snapped, connected, ran `sudo certbot plugins --prepare`
- added temporary changes to have both certbot and certbot-dns-dnsimple use DNSAuthenticator2
- snapped and installed certbot, `certbot plugins` failed as expected.
- snapped and installed certbot-dns-dnsimple, `sudo certbot plugins --prepare` succeeded
- Inspected dns plugin's `bin` and `lib`; no `certbot` or `acme`, as expected.
```
$ ls /snap/certbot-dns-dnsimple/current/lib/python3.6/site-packages/
OpenSSL future-0.18.2.dist-info requests_file.py
PyYAML-5.3.1.dist-info idna setuptools
_cffi_backend.cpython-36m-x86_64-linux-gnu.so idna-2.9.dist-info setuptools-47.3.1.dist-info
certbot_dns_dnsimple lexicon six-1.15.0.dist-info
certbot_dns_dnsimple-1.6.0.dev0-py3.6.egg-info libfuturize six.py
certifi libpasteurize tldextract
certifi-2020.4.5.1.dist-info past tldextract-2.2.2.dist-info
cffi pip urllib3
cffi-1.14.0.dist-info pip-20.1.1.dist-info urllib3-1.25.9.dist-info
chardet pkg_resources wheel
chardet-3.0.4.dist-info pyOpenSSL-19.1.0.dist-info wheel-0.34.2.dist-info
cryptography pycparser yaml
cryptography-2.8.dist-info pycparser-2.20.dist-info zope
dns_lexicon-3.3.26.dist-info requests zope.interface-5.1.0-py3.6-nspkg.pth
easy_install.py requests-2.23.0.dist-info zope.interface-5.1.0.dist-info
future requests_file-1.5.1.dist-info
$ ls /snap/certbot-dns-dnsimple/current/bin/
chardetect futurize lexicon pasteurize tldextract
```
- reset to HEAD^
- snapped and installed certbot to not have the DNSAuthenticator2 changes, `certbot plugins` failed as expected.
* Don't include certbot deps when EXCLUDE_CERTBOT_DEPS is set
* Set EXCLUDE_CERTBOT_DEPS in certbot-dns-dnsimple/snap/snapcraft.yaml
