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/plugins/TesterPlugin/plugin.yml

@@ -1,5 +1,6 @@
 name: TesterPlugin
 main: pmmp\TesterPlugin\Main
+src-namespace-prefix: pmmp\TesterPlugin
 version: 0.1.0
 api: [3.2.0, 4.0.0]
 load: POSTWORLD