I just backed off Xcode 13.0ß3 and moved back to ß2. I'm compiling again. BUT...
(1) I don't know why the local SPM packages should have been compiled with the APPLICATION_EXTENSION_API_ONLY flag. The flag is not set anywhere in the project.
(2) There have been a lot of problems with Xcode's implementation of SPM which are (at least partially) due to having absolutely no control over package compilation.
(3) Every time the Xcode team fixes something related to SPM, it seems like they either reveal or introduce new bugs. Based on the problems I've been having over the past year, I am having some serious issues with what aspects of the feature set are included in their test suites. (And please understand that I mean no disrespect. Xcode is an outstanding achievement. But I've reported a half-dozen or so common use-cases that don't work at all [and hence must not be included in their test suite.])
I wish I had an estimate of the amount of time I've wasted trying to use features which are purported to work regarding SPM. It's certainly at least 3 man-months. Probably more.
– frustrated in St. Louis