I know of no API guarantee about where the data can be found in the first place. If you cannot find it, you cannot copy it. Some experimentation allowed me to decipher the current implementation details to find it: BuildParamaters.codeCovPath + package name + ".json". But not knowing how stable that assumption is forces me to gate the functionality to a specific patch of Swift.
That could be solved either by the proposed ability to specify the output file location, or instead by some means to query the package manager about where it put the data (comparable to swift build --show-bin-path).
The JSON format also seems to be an implementation detail, so in the same vein, it would be nice if the package manager vended a library module capable of loading it. I have a library that does so, but since it requires relying on implementation details, it would be nice to sink such functionality into the SwiftPM package.