Issue Description:

Running swift package generate-xcodeproj should always produce the same output given no changes to the project. It currently emits different output every time, which leads to a lot of extra source control noise when re-running to capture a new file or two.

Steps to reproduce:

mkdir -p MyLib
cd MyLib
git init
swift package init --type library
swift package generate-xcodeproj
git add .
git add -f MyLib.xcodeproj
git commit -m "Test"
swift package generate-xcodeproj
git status

Expectation: git status returns no changes.
Actual: git status returns dirty Xcode project file.