I have this same problem but macOS (11.1) Xcode 12.0 (17219); which begin on Big Sur in app review, and since I updated and trying to triage what's new / different / required. Trying to launch host app via custom URL.

I got side tracked but with few exceptions, these sub-projects I had exported to GitHub from their origins when they were self contained within their original app I'm now returning to. All was nice and simple. I mused just putting them back, within the app, but that seemed like a bad move, backward, just like using an older system / build environment which last worked and just continue there. One sub-project in particular - CocoaAsyncSocket, appears to have made the grade to SPM, so I'm wading towards that path for the others(6). Aside from explicit dependency entry in build settings, the subject of header resolution leads me to think that perhaps I shouldn't have or use such explicit inclusions, as otherwise the SPM and its definitions (headers) should be treated like any framework - black box?

I neglected to mention this is MacOS, where resources re: the new view are pretty sparse, but thanks.