Find or create XSD for every XML file we deal with
As everyone is probably aware, the XML namespace used for yum repositories (http://linux.duke.edu/metadata/) is dead and gone. This is sad.
We deal with the yum metadata XML in this plugin, so we should have an XML Schema Definition (XSD) for the XML we accept. These XSD have probably already been written by someone, but we don't know where they are. We should find (or, as a last resort, write) an XSD for each file type. We should use them to validate incoming XML. This provides two benefits:
1) We know when upstream metadata doesn't match our expectations.
2) We document our expectations of the metadata format. This is helpful when we encounter a problem so people can either determine we are not expected the "correct" XML, or they are creating "incorrect" XML.