Answering my own question: The view/scene had a size of 1x1 pixels stretched out, that was the root cause.

Yes, but I always get to the same dialog presenting me all Xcode-internal Swift packages. If I hit the "Local ..." button at the bottom, I can again select the package from Finder, and a duplicate of the package shows up in the left-hand folder view under "Packages", nowhere else. The problem persists. Either Apple doesn't want me to use local packages (which would be kinda stupid) or this is a serious bug. Can't believe it is THAT hard to include local Swift packages. I am running Xcode 13.3.

Thank you ronm for the great help! As I found out now, it seems Xcode needs the Package being a Git repo in order to properly work, and the repo root directory has to be the root directory also for the package. (not in a sub-directory) As for now, this is fine with me, as I plan to version-control the package anyway. I first added the package via General -> Project -> Package Dependencies -> "+" Button And then added it on the target as you suggested via General -> Target(s) -> Frameworks, Libraries and Embedded Content.