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/git/receive-app

@@ -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"
 
 APP="$1"; REV="$2"