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

tests/phpstan/configs/check-explicit-mixed-baseline.neon

@@ -210,6 +210,11 @@ parameters:
 			count: 1
 			path: ../../../src/plugin/PluginDescription.php
 
+		-
+			message: "#^Property pocketmine\\\\plugin\\\\PluginDescription\\:\\:\\$srcNamespacePrefix \\(string\\) does not accept mixed\\.$#"
+			count: 1
+			path: ../../../src/plugin/PluginDescription.php
+
 		-
 			message: "#^Parameter \\#1 \\$plugins of class pocketmine\\\\plugin\\\\PluginGraylist constructor expects array\\<string\\>, mixed given\\.$#"
 			count: 1