Handle absolute config paths

[elliotdavies]

Description of the change

Replaces #485 as a fix for #472, and tidies up a few things.

Fix #472

Checklist:

• [x] Added the change to the "Unreleased" section of the changelog
• [ ] ~Added some example of the new feature to the README~
• [x] Added a test for the contribution (if applicable)

P.S.: the above checks are not compulsory to get a change merged, so you may skip them. However, taking care of them will result in less work for the maintainers and will be much appreciated 😊

[f-f]

Update: it looks something is up with the Windows tests

[elliotdavies]

@f-f Oh of course, because of the absolute paths on Windows... 🙈 I guess we'll have to do without for now?

[elliotdavies]

The tests are continuing to fail on Windows because I switched to using Dhall.Import.localToPath which returns an OS-specific path, but our test fixtures hard-code the Unix versions. @f-f do you know if it would be difficult to use a different text fixture on Windows, or something similar?

[f-f]

@elliotdavies yes, we can case on the OS, as we do already e.g. here: https://github.com/spacchetti/spago/blob/a4679880402ead320f8be2f091b25d30e27b62df/src/Spago/Bower.hs#L142-L146

[elliotdavies]

(I'm kind of blocked on this without a Windows machine of my own to test on. The fixtures certainly need updating but some of the other tests are failing too and I'm not certain why.)

[toastal]

Any updates on this? I want to use process substitution to create files on the fly, but I can't because of the .//tmp/absolute/directiory/spago.dhall issue.