Internal link targets aren't converted in any way

The target of an internal link is taken as is. This creates the following problems:

1. Relative links aren't resolved (syntax like ..:..:page)
2. The page id isn't cleaned (e.g., convert uppercase to lowercase or spaces to _)
3. DokuWiki namespaces are converted into XWiki spaces, but, e.g., a link to playground:playground (the page playground in the namespaces playground) will point to a page named playground:playground in the current space.
4. If a link contains an anchor like #heading, that anchor becomes part of the target page name instead of an anchor. See also DOKUWIKI-28 for the topic of mapping DokuWiki heading ids to XWiki heading ids.

Links should be resolved and cleaned and anchors should be at the very least extracted.