Revamp the linking strategy in Haddock.

Now name resolution is done in two phases:

 - first resolve everything to original names, like a Haskell compiler
   would.

 - then, figure out the "home" location for every entity, and point
   all the links to there.  The home location is the lowest non-hidden
   module in the import hierarchy that documents the entity.  If there
   are multiple candidates, one is chosen at random.

Also:

 - Haddock should not generate any HTML with dangling links any more.
   Unlinked references are just rendered as plain text.

 - Error reporting is better: if we can't find a link destination for
   an entity reference, we now emit a warning.