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/20_events/commands

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-source "$(dirname $0)/../common/functions"
+source "$PLUGIN_PATH/common/functions"
 
 PLUGIN_DIR="$(dirname $0)"
 

plugins/20_events/hook

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-source "$(dirname $0)/../common/functions"
+source "$PLUGIN_PATH/common/functions"
 
 [[ ! "$DOKKU_EVENTS" ]] || dokku_log_pluginhook_call "$(basename $0)" "$@"

plugins/20_events/install

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
-source "$(dirname $0)/../common/functions"
+source "$PLUGIN_PATH/common/functions"
 
 DOKKU_RSYSLOG_FILTER=/etc/rsyslog.d/99-dokku.conf
 DOKKU_LOGROTATE_FILE=/etc/logrotate.d/dokku