Build failure with combination of hs-boot files, preprocessors and --working-dir

[sheaf]

If you try to build a package which:

• contains an hs-boot file, say A.hs-boot
• where the module A is obtained by running a preprocessor, say A.hsc,
• and using --working-dir flag

then the build will fail with the error:

attempting to use module `A` (A.hs-boot) which is not loaded

The problem is that the runPreProcessorWithHsBootHack function does not correctly handle the working directory.

I have a fix in #10991.