Use Confluence resolvers to resolve pages at import time

To resolve links outside imported packages, it is currently necessary to provide a link mapping in parameters. This is not convenient to use when using confluence-xml from a generic interface like the Filter Streams Application, and this doesn't scale for huge migrations, where users need to merge big objects together and provide this huge merged object to confluence-xml.

The Confluence contrib project now has a framework to provide Confluence resolvers, which find pages from their space and titles, or from their page ids. We can use this to resolve pages at migration time.

A "Use Confluence Resolvers" parameter shall let users disable this on-by-default mechanism.