refactor: move all vhost related functions to the domains plugin

These should never have been in common, and more directly relate to domain handling than anything else.

As this is a BC-break, merging will need to wait until the next minor.

plugins/nginx-vhosts/proxy-enable

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
+source "$PLUGIN_AVAILABLE_PATH/domains/functions"
 source "$PLUGIN_AVAILABLE_PATH/proxy/functions"
 source "$PLUGIN_AVAILABLE_PATH/ps/functions"