Xcode package project complaining about non-empty directory

It's been a couple months since I worked on this project, and I probably upgraded from Xcode 11 to 12 in the interim. Opening the project just now, it failed (even without an explicit build invocation) with:

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An unknown error occurred. '/Users/me/Library/Developer/Xcode/DerivedData/MyProj-elecieykitymjscdsykarofselrg/SourcePackages/checkouts/swift-nio' exists and is not an empty directory (-4)

Any idea what this is and how to fix it (without potentially discarding local changes I might've made to the dependency)?

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