So I'm trying to add SPM support to this repo. As you can see they've got a fairly deep folder structure. I can get it all to work by using the method suggested above and adding symlinks to an include folder.
However, before I do a PR I'd like to not require the addition of so many files. I can't get publicHeadersPath to work, I'm thinking because everything is so nested.
I read (here) that using cSettings and setting .headerSearchPath should maybe work, but it seems like that would require lots of entries.
If the project is currently set up to copy the headers into the product and rearrange them, then you are probably stuck simulating that final arrangement with symlinks.
SwiftPM vends all sources directly and has no header copying phase, so relative paths must reflect the locations of files when they are first checked out.