Avoid 404 links to r-l.org docs for local pkgs.

In other words, `doc-uri` should return #f for locally-installed
packages that aren't part of the Racket main distribution and therefore
don't have docs on www.racket-lang.org.

Also, add typed/racket and typed/racket/base to the list of modules for
which we'll use their documentation as a "tie-breaker", just like racket
and racket/base.