Internal links with fragments are not captured by softlink indexer

Description

Steps to reproduce

1. Create a new page
2. Add a link to an existing page using an external link with a fragment, for example, http://<sitehost>/en/search#frag.
3. Publish the page.
4. Right click the linked page and select Move to trash.

Expected: Warning that search is linked from the page created in 1.
Actual: No warning.

Investigation shows that call to Uri.IsWellFormedUriString (in XhtmlStringIndexer.cs) returns false for relative urls when they contain #. Perhaps relative links should transform to absolute ones (temporary) before calling IsWellFormedUriString