Add a virtual "windows" package

[lazka]

Just a random thought.

We could add a virtual pacman package (at runtime, same as --assume-installed) "windows=10.0.22621" so we can depend on specific Windows versions and prevent updates. Users could override it by passing --assume-installed windows=999

At the same time we'd need a real windows package with a large version so users can update to it while satisfying all dep checks, which then gets overridden by the new pacman package with the virtual one, once it's there