Need mechanism to inject targets from package *before* common targets

Currently, a package can have logic that runs after common props or after common targets, but we also need to hook in after the users settings but before common targets.

For now, I'm setting `CustomBeforeMicrosoftCommonTargets`, but that's a hack that hijacks something that should belong to the user. We need to design a way for nuget packages to do this without stepping on anyone else.

cc @jeffkl