listNamespaceFiles: Ensure trailing slash

[MGatner]

Description
The Autoloader merges Composer namespace paths directly from Composer's autoloader, which doesn't include a trailing slash. This means files coming from autoloader->getNamespace() may or may not have a trailing slash, which sometimes causes FileLocator->listNamespaceFiles() to miss files it should otherwise find.
This change ensures that the namespace path coming from autoloader will always have the trailing slash.

Checklist:

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

[lonnieezell]

Oh, nice catch! Thanks.