fix: Composer PSR-4 overwrites Config\Autoload::$psr4

[kenjis]

Description
The user guide says:

If the same namespace is defined in both CodeIgniter and Composer, CodeIgniter’s autoloader will be the first one to get a chance to locate the file.
https://codeigniter4.github.io/CodeIgniter4/concepts/autoloader.html#composer-support

But if you move the app/ folder, the autoloader can't find Config\App.php only when Composer is enabled.
See https://forum.codeigniter.com/showthread.php?tid=81690

It is because the Composer's namespace paths overwrite the Config\Autoload::$psr4 settings.

CodeIgniter4/system/Autoloader/Autoloader.php

Line 331 in faff1f8

$this->prefixes = array_merge($this->prefixes, $newPaths);

If the input arrays have the same string keys, then the later value for that key will overwrite the previous one.
https://www.php.net/manual/en/function.array-merge.php#refsect1-function.array-merge-description

This PR fixes the bug.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[MGatner]

@lonnieezell had some issues early on with using the CodeIgniter namespace in some packages (hence "Sparks"). I assume this won't affect those since they have subsequent parts (e.g. CodeIgniter\Tasks) but something to keep an eye on.

[kenjis]

@MGatner If you mean #5292, I fixed.