Allow plugins to use PSR-4 namespace mapping (#4188)

* Allow plugins to use PSR-4 namespace mapping
this is a reduced implementation which serves the 99% use case without being horribly breakable.

Plugins may now specify a `src-namespace-prefix`, which should be set to the namespace of the classes in `src`.
If the old system is used, `src-namespace-prefix` can be omitted, or set to an empty string.

Examples:
- If `src-namespace-prefix` is `dktapps\test`, `dktapps\test\Main` will be searched for in `src/Main.php`, instead of `src/dktapps/test/Main.php`.

* Migrate TesterPlugin to PSR-4

src/plugin/PharPluginLoader.php

@@ -48,7 +48,10 @@ class PharPluginLoader implements PluginLoader{
 	 * Loads the plugin contained in $file
 	 */
 	public function loadPlugin(string $file) : void{
-		$this->loader->addPath("$file/src");
+		$description = $this->getPluginDescription($file);
+		if($description !== null){
+			$this->loader->addPath($description->getSrcNamespacePrefix(), "$file/src");
+		}
 	}
 
 	/**

src/plugin/PluginDescription.php

@@ -48,6 +48,7 @@ class PluginDescription{
 	private $name;
 	/** @var string */
 	private $main;
+	private string $srcNamespacePrefix = "";
 	/** @var string[] */
 	private $api;
 	/** @var int[] */
@@ -114,6 +115,8 @@ class PluginDescription{
 			throw new PluginException("Invalid Plugin main, cannot start within the PocketMine namespace");
 		}
 
+		$this->srcNamespacePrefix = $plugin["src-namespace-prefix"] ?? "";
+
 		$this->api = array_map("\strval", (array) ($plugin["api"] ?? []));
 		$this->compatibleMcpeProtocols = array_map("\intval", (array) ($plugin["mcpe-protocol"] ?? []));
 		$this->compatibleOperatingSystems = array_map("\strval", (array) ($plugin["os"] ?? []));
@@ -284,6 +287,8 @@ class PluginDescription{
 		return $this->main;
 	}
 
+	public function getSrcNamespacePrefix() : string{ return $this->srcNamespacePrefix; }
+
 	public function getName() : string{
 		return $this->name;
 	}