Update config() to check all namespaces

[MGatner]

Description
Calling config($classname) with an un-namespaced class passes the string to $locator->locateFile() which causes it to use legacyLocate(), which searches App and System only. This change leaves the default behavior but falls back to $locator->search() instead of failing immediately, allowing calls like config('Auth') to check first in App and System and then in any autoloaded namespaces.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide