fix(discovery): support PSR-4 paths defined as arrays in composer.json

[nayleen]

This reproduces the error I'm seeing in a project using tempest/discovery standalone.

The issue most likely affects the main framework as well due to a assumption in the Discovery Composer.php class that PSR-4 namespaces only map to a single path, even though Composer's schema allows both string and list<string>:

https://getcomposer.org/doc/04-schema.md#psr-4

If you need to search for a same prefix in multiple directories, you can specify them as an array as such:

{
    "autoload": {
        "psr-4": { "Monolog\\": ["src/", "lib/"] }
    }
}

As per the contribution guidelines I've only submitted the failing test, even though I gave a fix a shot already.

You'll find the patch in the following Gist: https://gist.github.com/nayleen/1696e073c98c98f7b822ed51927e3350

The test reproduces the exact error I'm seeing:

Tests\Tempest\Integration\Core\ComposerTest::supports_psr4_paths_as_array
TypeError: Tempest\Discovery\Composer::{closure:Tempest\Discovery\Composer::load():41}(): Argument #1 ($path) must > be of type string, array given, called in /app/src/packages/support/src/Arr/functions.php on line 862