Use $PLUGIN_PATH instead of $(dirname $0)/..

As briefly discussed in #1425, source the `common/functions` file via an
absolute path, rather than determining the relative path by using
`dirname`. 3rd-party plugins should follow suit and use the new
`$PLUGIN_PATH` convention too.

plugins/checks/check-deploy

@@ -35,7 +35,7 @@
 #
 
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-source "$(dirname $0)/../common/functions"
+source "$PLUGIN_PATH/common/functions"
 
 APP="$1"; DOKKU_APP_CONTAINER_ID="$2"; DOKKU_APP_CONTAINER_TYPE="$3"; DOKKU_APP_LISTEN_PORT="$4"; DOKKU_APP_LISTEN_IP="$5"
 if [[ -z "$DOKKU_APP_LISTEN_PORT" ]] && [[ -f "$DOKKU_ROOT/$APP/PORT" ]]; then