aasx: Fix handling of File objects which refer to external URLs

It's currently unclear, what kind of values a File object may contain and how they should be interpreted. Probably, they are allowed to hold a full URL which refers to some webserver etc. as an alternative to relative and absolute paths (./some_file.pdf resp. /some/path/to/some_file.pdf), which refer to a local file.

We currently assume that all File objects within an AASX package refer to a supplementary file within that package. Thus, reading an AASX package file fails when it contains a File object refering to an external file via URL.

To fix that, we should distinguish between local and external references. However, currently the semantics of the File.value attribute's values are not clear from the standard (DotAAS), so we can only guess that the existence of the URL schema is the correct criterion.