Enhance content validation for maven-metadata.xml files

The current file content validation for maven-metadata.xml files does very little. Caching bad metadata.xml files from servers can lead to build failures.  Furthermore, it can often be difficult to clean up bad maven-metadata.xml files from a proxy's cache, and even if you do, they may just get downloaded again.

Content validation for maven-metadata.xml files can be improved by parsing the files and making sure that:

1. The GAV coordinates metadata.xml file are present in the file
2. The GAV coordinates match the path the file is stored in

Files which do not meet the above criteria should be rejected.