When Jonathan was adding a new package to FX, we realized that he needed to add his:
    --add-export=javafx.graphics/xxxx=javafx.controls
to 5 different files, 4 of them the generated argfiles in ./build

If we add a mechanism to concatenate the contents of a source file, for example buildSrc/addExports to the four generated argfiles, then we only need to modify one file when creating a new package, and modify one file when the change has been promoted into the JDK.

The task is to create the mechanism, starting with close an addExports file that contains comments to start.