Support autoloading for relative namespaces

[caipre]

At present, hussar will not add require_once statements for classes referenced relative to the file's namespace. That is, if we never see the fully-qualified namespace to a class, it will be absent from the .class-index cache and inline greps, resulting in false-positive UnknownClass errors.

We can support this by including filenames in the grep and looking for a namespace declaration within locate_class_files.