Customisable location for haskell-mode-generate-tags output

Feature request.
As far as I understand from brief look at the code, `haskell-mode-generate-tags` walks upwards looking for the `.cabal` file from the project. That's fine but it would be nice to have a bit more freedom.

My use-case is that we have a project `foo` with multiple Haskell projects in it. Normally I run `hasktags` in `foo`, load the tags in and can jump to all the Haskell definitions under `foo`, not just the project I'm working in. It's really useful when multiple projects with dependencies between each other live close by.

It'd be nice to be able to specify somehow our own function to find the directory to use for TAGS, using `haskell-cabal--find-tags-dir` as the default. This would allow the user to write a small elisp to handle such a special scenario.

Start at the haskell-mode-generate-tags entry point and read how it uses haskell-cabal--find-tags-dir to locate the TAGS directory. Define what user-provided directory-finding behavior should look like while preserving the existing function as the default, then verify that tags can be generated from the selected location.

Written by the indexing model from the issue text.