"the Package.resolved file is most likely severely out-of-date and is preventing correct resolution"

I'm getting the following error when trying to build. It shows up as the only item in the build log, too.

Here are the packages I'm using:

5 Swift packages listed in the project settings: SimpleLogging, OptionalTools, FunctionTools, SpecialString, and Swift-TODO. The first 3 are URLs to git files (they end in .git), and the last 2 are URLs to the repos themselves (they end in a slash). The same 5 packages as above, but listed in the sidebar, proving they were resolved successfully.

Solution

For me, it turned out that, even though Xcode was able to resolve and clone these packages, it didn't like that 2 of them pointed to the GitHub repo page, rather than the git file version.

Changing them so they're all .git URLs fixed it:

The same project package list as above, but none end in a slash; all end in .git