rollup-app-reexports writes package.json even if not needed

[pieter-v]

We use perforce as a source control system. All files which you are not modifying are read-only.
So the build fails because rollup-app-reexports writes the file package.json even if the result is the same as the original content (https://github.com/embroider-build/embroider/blob/1ecd57ea20195cd54358debde5801f82611d81d1/packages/addon-dev/src/rollup-app-reexports.ts#LL38C19-L38C19)

So, it would be nice if rollup-app-reexports would only write a source file if needed

[NullVoxPopuli]

Thanks for the report @pieter-v !! I put a PR resolving this issue for app-reexports as well as keep-assets

#1423