We migrated our plain Xcode project over to SPM. Compared to the normal project SPM does not allow overlapping sources or in other words multiple targets referencing the same root source. In our case that's exactly what we needed, at least until we figured out how to refactor our huge code base and get rid of all defines. That said, our code base merges multiple targets and uses defines to produce different results. This is possible to replicate in SPM by creating multiple targets and applying custom defines and it also compiles as expected. However only Xcode cannot properly resolve the related symbolic links, which are used to overcome the overlapping root source code path.
In fact, such solution was suggested in this post here:
Note that the targets with overlapping sources are final application products, not libraries, which is why it should be totally fine allowing their source code to overlap. However they are still marked as libraries so that we can reach their entry point from a single file in the actual Xcode project to jumpstart the final application.