Fix urls

Lots of files touched, not actually changed

1) drop obsolete dsource references
2) switch ddoc to point to dlang.org
3) drop paths in references to backendlicense.txt
4) update readme.txt to point to dlang.org

I left the reference to digitalmars.com in the copyright headers since digitalmars holds the copyright and those urls reference the digitalmars site as a company rather than as the site to go to for d info.