`hoogle generate --local` ignores `haddock-html` field

Tested at commit e11273ec17c54f83344861e332580996f9f4127e.

When I try to generate a Hoogle database on NixOS (in a Nix shell that provides GHC, Cabal, and Hoogle), I get the following error:

$ hoogle generate --local --database db.hoo
Starting generate
Reading ghc-pkg... 0.03s

Packages missing documentation: array base binary bytestring Cabal Cabal-syntax containers deepseq directory exceptions filepath ghc ghc-bignum ghc-boot ghc-boot-th ghc-compact ghc-experimental ghc-heap ghc-internal ghc-platform ghc-prim ghc-toolchain ghci haskeline hpc integer-gmp mtl os-string parsec pretty process rts semaphore-compat stm system-cxx-std-lib template-haskell terminfo text time transformers unix xhtml
No packages were found, aborting (use no arguments to index all of Stackage)

I found that the readGhcPkg function made too many assumptions about the location of package documentation.

I have a fix here, but I think the code could be improved.

Start by tracing the readGhcPkg function used by hoogle generate --local and reproduce the NixOS command from the issue. Check how ghc-pkg entries expose haddock-html, then verify that generation discovers the installed package documentation instead of aborting with no packages found.