Fix plugin loading from CWD

Currently packer can detect plugins in current working directory, but will fail when actually trying to load those plugins. 

This is because the discover mechanism will register a CWD plugin as just it's filename e.g. "packer-builder-foobar". However the plugin loader expect an executable path like "./packer-builder-foo".

This fixes it by passing the absolute path to CWD insto discover instead of ".", which again will lead it to store plugins with an absolute (and executable) path.