Add multi-path support to locateFile()

[MGatner]

Description
Currently locateFile() flattens namespace paths to a single path per namespace (see #1866). This fix searches all namespace paths for the first matching file.

This change also removes the auto-capitalize of namespaces as this is not a namespace requirement and could cause locateFile to fail to match valid file namespaces.

Checklist:

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