Alec Theriault authored Jan 19, 2019 and Ben Gamari committed Jan 23, 2019

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.