* 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
I am eating my own words this once, because having the tester plugin as a separate repository makes no sense - it is just added barriers to writing proper tests with no actual benefit. Since the tester plugin is specifically intended for CI, it doesn't make sense for it to be in its own module.
