Support hoogle for prebuilt and local packages

Atm, there is no explicit support for hoogle in rules_haskell, and no obvious way to generate a hoogle database for a given set of packages:

• Since haskell_doc runs haddock with the --hoogle flag, we can generate a hoogle db for local packages using something like hoogle generate $(ls bazel-bin/docs/ | sed 's/^/--local=bazel-bin\/docs\//') (assuming we have a haskell_doc rule named docs).

• We can also generate a db for the prebuilt dependencies using hoogle generate --local with the right ghc in scope.

The problem is that there is (afaik) no way to merge these databases nor an easy way to combine these two calls, so no way to create an haddock database containing all the available packages.

Start with the haskell_doc rule and the example docs target described in the issue, then inspect how rules_haskell handles prebuilt dependencies and Hoogle generation. Define a workflow that combines Hoogle data from local and prebuilt packages; done means one generated database contains both sets of available packages.