fix: is_plugin_active doesn't work when plugin is in mu-plugins

fixes #2

https://core.trac.wordpress.org/ticket/24205

src/Plugin.php

@@ -94,6 +94,14 @@ public function getContainer(): Container
         return $this->container;
     }
 
+    /**
+     * Checks if the plugin is active.
+     */
+    public function isActive(): bool
+    {
+        return is_plugin_active($this->container['plugin_basename']) || preg_match('#mu-plugins/[^/]*/ymir\.php$#i', $this->filePath);
+    }
+
     /**
      * Checks if the plugin is loaded.
      */

ymir.php

@@ -34,6 +34,6 @@
 $ymir->load();
 
 // Load Ymir pluggable functions if the plugin is active
-if (is_plugin_active(plugin_basename(__FILE__))) {
+if ($ymir->isActive()) {
     require_once __DIR__.'/pluggable.php';
 }