Defer pluginHooks() to the 'plugins_loaded' hook (#25)

This ensures that pluggable functions are available when the activation hooks are fired.

The priority is -9999 so that the activation hooks likely fire before any other functionality the plugins are attempting.

fixes #24

src/Autoloader.php

@@ -64,7 +64,7 @@ public function loadPlugins()
  include_once WPMU_PLUGIN_DIR . '/' . func_get_args()[0];
  }, array_keys($this->cache['plugins']));
 
- $this->pluginHooks();
+ add_action('plugins_loaded', [$this, 'pluginHooks'], -9999);
  }
 
  /**
@@ -134,7 +134,7 @@ private function updateCache()
  * loaded as usual. Plugins are removed by deletion, so there's no way
  * to deactivate or uninstall.
  */
- private function pluginHooks()
+ public function pluginHooks()
  {
  if (!is_array($this->activated)) {
  return;