Autoloading on plugin add.

> This is a minor commit, but it helps creating more consistent plugin `load` files. 

This is our current plugin loading strategy:

```fish
# Add all defined plugins
for plugin in $fish_plugins
  _fish_add_plugin $plugin
  _fish_add_completion $plugin
  _fish_source_plugin_load_file $plugin
end
```

If your plugin requires another plugin to function, you should be able to do so inside your `myplugin.load` like:

```fish
_fish_add_plugin plugin
```

This works for simple plugins with only a `plugin.fish` function file, but will fail to source the plugin's load file which is how additional dependencies are defined / required in a per-plugin basis.

`_fish_add_plugin` should automatically source load files and completions.