Don't include certbot deps when EXCLUDE_CERTBOT_DEPS is set in plugins (#8091)

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

certbot-dns-dnsimple/setup.py

@@ -12,12 +12,19 @@ version = '1.6.0.dev0'
 # Remember to update local-oldest-requirements.txt when changing the minimum
 # acme/certbot version.
 install_requires = [
-    'acme>=0.31.0',
-    'certbot>=1.1.0',
     'setuptools',
     'zope.interface',
 ]
 
+if not os.environ.get('EXCLUDE_CERTBOT_DEPS'):
+    install_requires.extend([
+        'acme>=0.31.0',
+        'certbot>=1.1.0',
+    ])
+elif 'bdist_wheel' in sys.argv[1:]:
+    raise RuntimeError('Unset EXCLUDE_CERTBOT_DEPS when building wheels '
+                       'to include certbot dependencies.')
+
 setuptools_known_environment_markers = (StrictVersion(setuptools_version) >= StrictVersion('36.2'))
 if setuptools_known_environment_markers:
     install_requires.append('mock ; python_version < "3.3"')

certbot-dns-dnsimple/snap/snapcraft.yaml

@@ -15,6 +15,8 @@ parts:
     override-pull: |
         snapcraftctl pull
         snapcraftctl set-version `grep ^version $SNAPCRAFT_PART_SRC/setup.py | cut -f2 -d= | tr -d "'[:space:]"`
+    build-environment:
+      - EXCLUDE_CERTBOT_DEPS: "True"
 
 slots:
   certbot: