Added vapor core swift package to my project so that I can use some of the extensions, had to `rm -rf ~/Library/Developer/Xcode/DerivedData/` to make xcode realise the repo is actually exist.
Then I set the Core package as dependency of one of the targets, XCode complaints "unknown package Core", ok fine then I remove Core dependency, Core complaints "dependency Core is not used by any target".
How can xcode comes to this contradicting conclusion is out of my imagination, it just never gonna be happy about however i write.