Hadrian: install patches 'haddock-{html,interface}'
Since the $(docdir) can be picked independently from the $(libdir),
we need to make sure that that the haddock-html and haddock-interface
fields in the package DB (which is in the $(libdir)) get updated to
point to the appropriate places in the $(docdir).
NB: in the make system, ghc-cabal would cover this sort of thing by
re-running configure on installation, but here we get away with a
couple lines of sed and a call to ghc-pkg recache.
Fixes #16202 (closed).