Auto-generate parts of the PackageOverrides.txt file

[ViktorHofer]

The checked-in PackageOverrides.txt file can easily become stale when a new library that also ships as a package gets added to the shared framework. We should look into auto-generating parts of PackageOverrides.txt or add validation to avoid a stale list.

[merriemcgaw]

Needs to be looked at so that it can be done in Preview 7.