haskellPackages: add hoogleLocal

The docs in `hoogle.nix` intend for it to be in the packageset as `hoogleLocal`
and that also makes a lot of sense from a user perspective.

`packages` is not a function `HaskellPackages -> [Package]`, but rather an
arbitrary set of packages.
That is good, because that way it can be used e.g. to merge package databases
from other versions of the packageset.

It’s done as function with default argument, because otherwise the user has to
use `overrideCabal` to override `packages` in `hoogle.nix`.