core: sets plugin_path to loaded plugins

core: sets plugin_path to loaded plugins

While loading a plugin in the manager using #load_botmodule_file this
will remember the directory of the file that is being loaded in
@next_plugin_path. In the plugin/BotModule initialization we set the
path in plugin_path of the plugin.

This was the only solution I could find to get this information in the
plugin. It is useful because this allows the plugin to know in which
directory it is located, it can then easily access data files stored
alongside the plugin.

Some built-in plugins (see #42) use the data/rbot/templates to copy
data files on bot load (see #repopulate_botclass_directory) into the
profile directory (~/.rbot/).