Added support for retrieving uri from referenced_document when uri is external file

[ketoketo]

Added support for retrieving uri from referenced_document when uri is an external file name.

Use case

XML

<ds:Reference URI="test.xml">
  <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
  <ds:DigestValue></ds:DigestValue>
</ds:Reference>

I encountered an XML with an external file name in the URI, and I was able to solve it by putting it in the hash of referred_documents and retrieving it by URI name, so I fixed it that way. The process to get them from referenced_documents already existed, so I made it into a function and reused it.

[jgg01]

This would help a use case we have as well @ketoketo , thank you