fix isolated-apache/nginx tests

it's a bit ugly, but by isolating just the paths of their internal
modules, this effectively replicates us testing the old plugins

tox.ini

@@ -145,7 +145,7 @@ deps =
     sakuracloud: -e certbot-dns-sakuracloud
     nginx: -e certbot-nginx
 commands =
-    apache: {[base]pytest} certbot-apache
+    apache: {[base]pytest} certbot/src/certbot/_internal/apache
     cloudflare: {[base]pytest} certbot-dns-cloudflare
     digitalocean: {[base]pytest} certbot-dns-digitalocean
     dnsimple: {[base]pytest} certbot-dns-dnsimple
@@ -159,7 +159,7 @@ commands =
     rfc2136: {[base]pytest} certbot-dns-rfc2136
     route53: {[base]pytest} certbot-dns-route53
     sakuracloud: {[base]pytest} certbot-dns-sakuracloud
-    nginx: {[base]pytest} certbot-nginx
+    nginx: {[base]pytest} certbot/src/certbot/_internal/nginx
 
 [testenv:apacheconftest]
 deps =