The msbuild pack target should have a public hook for running user targets before itself

[stephan-tolksdorf]

The Build and Clean targets have public subtargets BeforeBuild and BeforeClean that can be used to ensure that custom targets are evaluated before the Build or Clean target, see https://docs.microsoft.com/en-us/visualstudio/msbuild/how-to-extend-the-visual-studio-build-process

The Pack target currently lacks such a subtarget. _IntermediatePack could be used within a BeforeTasks attribute to reliably hook into the beginning of the pack job, but the name of that target suggests that it is a private implementation detail.