Take working directory into account in preprocessor hs-boot hack

[sheaf]

This commit fixes an oversight in runPreProcessorWithHsBootHack, which did not correctly take into account the working directory. This could lead to situations in which we failed to apply the hack.

Recall that this hack is for situations when one has both an hs-boot file and a file in need of preprocessing, such as:

  Foo.y
  Foo.hs-boot

or

  Bar.hsc
  Bar.hs-boot

Failing to apply the hack essentially meant that GHC would not be able to see the hs-boot file, which caused build failures.

---

Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added.

[sheaf]

I added a test and changelog entry. This is now ready for review.

[mergify]

This pull request has been removed from the queue for the following reason: pull request branch update failed.

The pull request can't be updated.

You should update or rebase your pull request manually. If you do, this pull request will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.