Javadoc
When the imported schema location has been resolved through catalog, we need to:
1) get a valid relative location to use for recursion into the imported schema
2) add an entry to the doneSchemas map using such a valid relative location, as that's
what will be used later for import links
The valid relative location for the imported schema is computed by first obtaining the
relative uri that maps the importing schema resolved location into the imported schema
resolved location, then such value is resolved on top of the valid relative location
that's saved in the doneSchemas map for the importing schema.